Information

Minecraft version: 1.19.2 (actually any latest version that you were working at the moment, and, if not much overkill, 1.12.2)
Modloader: Fabric (for modern versions, if doing for 1.12.2, Forge)

Feature description

I know this can be a bit more complex compared to another mods that you've made, but at the same time i felt not only this concept didn't get much attention by the modding community but also this somewhat fits with the "vanilla+" feel of your mods.

Basically i'm thinking in a mod that revamps the mechanics of minecarts, allowing it to create meaningful railroads with them. In mind i have as a reference the plugin TrainCarts, originally for Spigot/Bukkit/Paper.
By the fact Bukkit and derivatives can't historically implement new blocks and items alone, TC ends up with a very "vanilla+" feel, using existent blocks and items to achieve this functionality.

Basically the idea is to allow minecarts to be "linked" together, forming a train, that is basically a group of minecarts acting as a single entity, with harmonious physics - ie. linked minecarts doesn't bump each other and cancel the general movement of the train, and players (at least non-ops) cant actually change the movement of the train, acting like its a "heavy" object. This specifically is a very recurrent issue on any native Fabric/Forge mods that implements the minecart linking feature solely: it doesn't change the physics, so the trains is very unlikely to stay running straight for longer periods, since in any turn half of it keeps running and the other half bump in the front carts and turn back detaching from the rest of the group. The linking can be made in a similar way we leash vanilla entities (basically "tying" a minecart to another), maybe even using the lead item for this, althrough i would suggest the use of the chain item to be more aligned with the thematics of the mod.

That said, besides the minecart-linking feature, TC also implements an automation system using signs - since Bukkit plugins couldn't historically implement items/blocks to the client, as i stated above. Although it's a blessing to automate railroads, i don't think this is something much critical to be implemented, if you think its getting too much complex for a mod. Having the train with a unified/harmonized physics, maybe the vanilla redstone system can be used to automatically control a station.

Also, trains (i don't think single minecarts have this) also can (configurable) load chunks that they are in and/or heading to. This is something very critical for autonomous railroads because if no players are loading chunks in the whole railroad it will simply fail when the trains heads to unloaded areas, making it difficult to create long-distance railways. There is standalone mods that implements this, but having this option built in and only for trains can be more vanilla-friendly - and minimize the possibility of incompatibility since both would changes concurrently the minecart item/entity.

So, sorry if this request is too big. I really think the minecart aspect of Minecraft is very much subutilized and could be a nice thing to see more mods revamping it, and turning to a useful transportation system.