I made an entire thread on what I think SRB2 v2.3 could feature.
But I'll summarize it here in case you don't feel like reading the entire thing, as well as add some new ideas.
- Minor changes to GFZ, THZ, and ACZ.
- All boss arenas getting an upgrade in visuals.
- DSZ2 remade, CEZ2 finished, RVZ2 and RVZ3, and a completely remade ERZ.
- Dark City Zone and Grand Eggship Zone seeing the light of day for the first time.
- A 10th zone in the form of Frozen Hillside Zone, as there's 2 existing acts.
- A Super Sonic level akin to Doomsday Zone from Sonic & Knuckles, with a new boss called the Egg Gunner. (more details on the thread)
- Tweaks (mostly minor and some major) to all seven Special Stages, and reintroducing clairebun's music_remade tracks.
- Spring Hill revival, Christmas Chime remade, DHZ and APZ minor changes, and the removal of Black Hole Zone.
The changes I want for multiplayer are bigger than this, so I'll put them in their own section.
- Weapon Rings : Homing Ring revival (with new behavior), Infinity Ring removal, and a new weapon ring called the Precision Ring, basically an upgraded Homing Ring.
- Match Super forms being brought back, and characters without Super sprites have a Super shield surrounding them.
- The cut multiplayer maps being brought back, taking advantage of 2.2's visuals, as well as new maps from past OLDCs.
- Battlemod in vanilla, with new maps along with it.
- Backwards compatibility with 2.2 mods, and mobile support restored. (PLEASE, SONIC TEAM JUNIOR, PLEASE GIVE US MOBILE SUPPORT AGAIN, PLEASE!)
Now for some new ideas that aren't from the thread, such as new characters and new enemies.
- Maybe add three new characters to the roster, all unlockable like the ones added in 2.2. These characters are Mighty the Armadillo, Ray the Flying Squirrel, and Espio the Chameleon, who are all character mods on the Message Board already.
- At least one new badnik in each zone. It's up to you on which ones you want.
- Definitely a new color palette, as the one we have is bland as hell.
- Adventure mode brought back. It's already a mod, so I don't see why not.