I have some more suggestions.
1.) A constant-run sector. In S3&K, Mushroom Hill Zone 2's boss used this.
You couldn't stop running, and couldn't face the other way. However, in that battle, you could speed up and slow down. Could work like this: A miscellaneous linedef action tagged to a sector with, possibly, the same effect. Linedef length is speed (minimum is 15, maximum is 55) and direction is running direction. If No Climb is set, use walking frames instead of running frames. If Effect 4 is set, the player is unable to jump. If Block Monsters flag is set, the player cannot speed up or slow down.
2.) Egg Rock Zone feels "too easy." The whole zone, not just one act. Maybe because it's unfinished, but there should be something that attracts rings. What I'm trying to suggest is a Jetty-Syn Bomber-esque badnik with a magnet on the bottom. This badnik flies above the player in various parts of the zone. When the player gets hit, they lose rings, right? The rings would fly upward onto that badnik's magnet. When said badnik is destroyed, the rings will fall off the magnet. If you touch the magnet, you will get the rings that you lost. BUT, the robot will only be able to carry 50 rings. This way, nobody can abuse "SETRINGS 9999" to keep depositing 32 rings into the enemy, then pop it and crash the whole server. It's not the best idea, but it is kind of great. Also, it would be comical to see n00bs screaming for us to kill it... XD
3.) When Super Sonic is flying on water, possibly have bigger waves appear. In Sonic 3's intro, Super Sonic flew across the water, and huge waves appeared. Basically, make big waves... Not important, but it feels wierd to see W-shaped splashes behind him when, in Colors, Unleashed, and other modern games (Which SRB2 isn't based on...) there's a / \ -shaped wave on both sides.
4.) A S3&K "mode." You already have pw_lightningshield and others (which are defunct), so why not make a mode for them where Sonic has no momentum change when he thoks, plays the Insta-Shield sound, and spawns an invulnerability sparkle (Object #163) and gets 15 tics of invincibility?
5.) Also, in every classic Sonic game (2, CD, 3, & Knuckles, and 4 if you count it,) Sonic had a dust cloud behind him when revving a Spin Dash. Maybe you could add it to SRB2? It could be easily Sonic Object Configured. Also, maybe a SPINREVITEM and/or GHOSTSPINREVITEM? Three changable objects exist in SOC Editor, and they are thok item, spin item, and a spin-rev item.
I think these would make SRB2 better, but if you don't want to put them in, that's fine with me.