My suggestions are as follows:
/speed:

• Able to /speed <1-10]>
  • ex. /speed fly 4
• /speed <1-10> [player]

/fly:

• /fly [player]
• This one is just to check for the extra argument of a player that is connected to the server to toggle their flight.

/vanish:

• Ditto of /fly
• Hide player(s) in list: https://www.minecraftforge.net/forum/topic/63425-hide-player-189/
• BUG: When I teleport to someone that didn't have me in their render distance while I was vanished they can see me.
• Fake leave/join messages

I know this is a lot, but I'm also in no rush, it's just things to think about.