I found this mod https://modrinth.com/mod/essentials-commands with some amazing utility commands, but its for Forge and I don't use Forge and I want these commands on Fabric, so here I am.
Here's a list of commands you can develop overtime, and remember of course you don't have to do any of these, I'm just giving ideas.

📌 This suggestion has been revamped and has now 26 new commands added and 11 unnecessary commands removed. You can still view the old list here: https://gist.github.com/User4716/91fdf30b40c3d6804c75d89edc75ab63

And yes, I think that the list below might still contain some unnecessary commands, but I left them in because the thing is that there is someone in a specific scenario who will find it useful :)

• /god <player> Enables/Disables God Mode
• /skin [player] [skin] Changes the skin of the specified player.
• /unskin [player] Resets a player's skin
• /day Sets time to day
• /night Sets time to night
• /sun Sets weather to clear
• /rain Sets weather to rain
• /thunder Sets weather to thunder
• /repair [durability] Fully repairs the tool in your hand unless a specific durability amount is specified
• /mute [player] [time] Mutes a player for specified time. Perm mute if no time specified
• /unmute [player] Unmutes a player
• /suicide A stripped down /kill command that anyone can use to only kill themselves
• /top Teleports you to the surface
• /trough Teleports you trough a wall you're facing
• /gmc [player] Alias to switch anyone's gamemode to adventure
• /gms [player] Alias to switch anyone's gamemode to adventure
• /gmsp [player] Alias to switch anyone's gamemode to adventure
• /gma [player] Alias to switch anyone's gamemode to adventure
• /pmtoggle [player] Toggles the visibility of all private messages to the specified player. If no player is specified then it would toggle private message visibility for you.
• /world [player] [world] [coordinates] Teleports the player to the specified world in the optionally specified XYZ
• /sudo [player] [command/msg] Force a player to send a message in the chat or execute a command, even if the player has no permission to execute it.
• /titletext [player] [text] An easier way to display title messages to any player. (With color code support)
• /subtitletext [player] [text] An easier way to display subtitle messages to any player. (With color code support)
• /actionbartext [player] [text] Allows you to display actionbar messages to any player. (Supports color codes)
• /feed [player] Feeds hunger and saturation levels, also resets exhaustion levels
• /heal [player] Heals you
• /armorpoints [player] [amount] Allows you to "wear armor" without actually wearing armor, basically edit armor levels
• /hungerpoints [player] [amount] Sets the hunger level for the specified player from 0 to 20
• /tempban [player] [time] Temporarily bans a player. The vanilla unban command can unban tempbanned players.
• /rename Rename the item in your hand, supports color codes.
• /hat [item] [player] Places the specified item to the specified player's head. If no player is specified, it will default to you. If no item is specified, the item in your hand will be the item that's placed on your head.
• /spawnentity [entity] [amount] [coordinates] [world] [interval] Spawn any amount of any entity to any coordinate in any world with any interval in ticks. (Try to optimize this so that it wouldn't try to spawn all entities at the same tick by default.)
• /extinguish [player] Cools any player off if you're on fire
• /fullbright [player] Toggles fullbright for any player
• /rotation [player] [yaw] [pitch] Allows for advanced head rotation
• /speed [0~10] [walk/fly/swim] [player] Changes the speed of a player or entity If second argument unspecified the mod will decide on what speed it will change depending if you are flying, swimming or on the ground.
• /tpoffline [player] Teleports to the last known location of a player
• /vanish [player] Hides a player's Skin, Armor, Hand items, Nametag, TAB name, Name from serverlist and Name from command arguments.
• /reply [message] Replies to the player you got a message from. You can also use /r [message]
• /msgtoggle [player] This can be used to toggle "msg mode" with a specified player. All messages you send in the chat will be sent as a msg to the specified player until you turn this off.
• /socialspy [player] Toggles socialspy for specified player, if player is not specified you'll see all private messages.
• /anvil [player] Opens an anvil GUI to the specified player
• /enderchest [player] This mod already has this command, but you should be able to open the enderchest of any player.
• /seen [player] Shows when was the last time the specified player was on the server
• /togglekeepinv [player] [world] Toggles keepinventory for a specified player in a specified world. If no player specified it will toggle it for everyone. If no world specified all worlds will be affected.
• /clearlag [radius] Kills all dropped items, ignited TNT and falling blocks with an optional radius in blocks. You can add more entities to be killed if you want in the config files. This command might same someones server from crashing if a griefer (or a bored admin) has worldedited 100,000 TNT and ignited it
• /nick [player] [nick] Changes the nick of the specified player.
• /unnick [player] Unnicks a player.
• /invsee [player] View and edit a player's inventory, even offline players.

‎

🪄 Additionally when playing on a multiplayer server where you do not have operator status, your client should not show operator-only commands or any commands that wouldn't work in the server in the TAB completion list. This function could be turned off if you wish to see the commands that this mod adds even if you can't execute them. But the config will by default hide these commands.

🪄Every [player] argument will not only show a list of all the online players in the TAB completion menu, but also the entity selectors that you can find in any vanilla command (@p, @s, @e etc...)

‎
💝I hope you didn't get overwhelmed by the amount of these commands, but if you do feel overwhelmed or pressured to do all this, you're taking this the wrong way. I spent a lot of time coming up with this list of ideas, and I'm only presenting this list to you for you to just look at it! And maybe you'll find a few of them useful and consider making them into reality. I'm sure there are many people who would appreciate many of these commands available on Fabric servers. And when you have the time and motivation to improve this mod, you could just start one by one making the commands you feel are the most important ones. This way I'm sure this mod will be a necessity for all modded servers in just a few months! 😉
‎

v2.3

+1 for mute, sudo, and suicide.
I believe /trash (it's called wastebin) and /top has already been implemented.

I removed the /trash command from the original suggestion

could you also add /msg (message user), /r (reply to the last person you sent a message to) and /spy (view other people's whispers)

• /god [Enables/Disables God Mode]
• /day [Turns day time]
• /sun [Turns day time]
• /night [Turns night time]
• /repair [Repairs the tool in your hand]
• /mute [Mute a player] (probably out of scope) (see banhammer)
• /unmute [Unmute a player] (probably out of scope) (see banhammer)
• /suicide [Alternative to /kill]
• /top [Teleports you to the surface]
• /survival [Survival mode] // CA
• /creative [Creative mode] // CA
• /spectator [Spectator mode] // CA
• /world [Teleport through dimensions] // CA
• /thunder [Strike a thunderbolt] (probably out of scope)
• /sudo [Force a player to execute a command] (idk about scope)
• /fireball [Shoot a big fireball] (probably out of scope)
• /customtext [Lets you create custom text with colors] (porbably out of scope) (see Styled Chat)
• /feed [Fulls your health and hunger bar]
• /heal [Fulls your health and hunger bar]
• /tempban [Temporarely ban someone] (probably out of scope) (See BanHammer)
• /nuke [May death rain upon them.] (probably out of scope)
• /rename [Rename the item in your hand] eh, could be something
• /hat [Wear the item in your hand] (there's mods for this)
• /egive [Give up to 3000 items to a player] (why?)
• /spawnentity [Spawn up to 100 mobs] (why?)
• /mobspawn [Spawn up to 100 mobs] (why?)
• /esummon [Spawn up to 100 mobs] (why?)
• /wear [Wear the item in your hand] (hat?)
• /furnace [Opens a furnace]
• /extinguish [Cools you off if you're on fire]

O.o github? I can edit someone else's list. Not sure if that's cool or terrifying lol

Anyway, some comments:

• fireball, thunder - I probably wouldn't reject it, but it'd be disabled by default, and I'm not personally going to implement it. *note: not OK with terrain-destructive (Note, you can /summon these)
• nuke - no
• sudo - This is just /execute? I think it would be a bad idea to try to remake that..
• spawnentity - multi-/summon I get it. Not opposed.
• egive - /give exists?

mute, unmute - EC doesn't aim to be a moderation tool

+1 to the rest of arno's comments, ty for going through those
full agree on the rest from arno

Can someone explain to me how a /sudo would differ from /execute? (I ask because it was mentioned a second time and got thumbsups)

/sudo would not just be a /execute alias. One difference would be for example:
/sudo orangc summon minecraft:cow
Which would summon a cow at the location of orangc.
Using /execute with this though would just summon the cow at the location of the executer, even if you used relative locations like this:
/execute as orangc run summon cow ~ ~ ~
I might be wrong though.

Yeah, I'm sorry I was lazy when I made this suggestion. I didn't have time to go through all of the commands and remove the unnecessary ones

Can someone explain to me how a /sudo would differ from /execute? (I ask because it was mentioned a second time and got thumbsups)

/execute at name run summon cow

/execute at name run summon cow

Good point, I feel like there's something I'm missing.
Other than this:
What about summoning the cow ~ ~ ~50 instead of ~ ~ ~?
I'm forgetting something important though 🤔

Further reading (the docs on /execute are actually quite nice, and come with examples): https://minecraft.fandom.com/wiki/Commands/execute#at

Fundamentally, you can think of /execute as changing the context of the command you run with it. This means /execute at <player> run <command> runs the command as if the command sender had the position of <player>.

Knowing that, to summon the cow 50 blocks +Z of the target player (I assume that's what you want with ~ ~ ~50), you would do: /execute at <player> run summon cow ~ ~ ~50. To summon it a few blocks above their head, similarly: /execute at <player> run summon cow ~ ~5 ~.

I updated the list and actually went trough all of them, removed unnecessary ones, and added a little too many new ones to it.
At least you won't run out of ideas 🙄

Also, a little disclaimer, I understand that you said that "EC doesn't aim to be a moderation tool" but I couldn't resist but to just leave /mute, /unmute and /tempban in the list :>
I know most people will use a plugin to moderate servers, but if someone can only have mods their server this would be a lifesaver.

And finally, the only way we could make the /sudo command useful is to actually make it force players to run the command, ignoring their operator status. You see where I'm going?
With this you could sudo a non-OP player to for example go to creative, or do anything. Your imagination is the only limit now!
(And this is not unheard of, a plugin managed to accomplish this simple goal of ignoring the permission check when running a command, so a mod should also probably be able to do it.)
This would allow for some crazy things and creations 👍

you forgot message and reply

Thanks for letting me know, I added /reply and /msgtoggle commands to the list. (Also an alias /r)
Minecraft already has a /msg command so I didn't see a reason to suggest that command. Could you tell me why would you want that?

PS. For the owner of this mod, I added a little ending speech to the original suggestion for you, so you won't take this huge list of commands the wrong way :)

you forgot message and reply