Mystic Realm includes:

Emerald collection in Mystic Realm 4 is vastly different from SRB2. Each of the seven main zones has a Mystic Temple in Act 2. Activating the Mystic Temple unlocks a hidden one act zone with plenty of new material. At the end of the hidden zone is a chaos emerald and the end level sign, taking the player back to Act 3.

Another feature in Mystic Realm are the emblems. Unlike SRB2 itself, the emblems are a fun-only feature, which should eliminate a lot of the cheating involved, since you don't get anything for getting them all except a ton of bragging rights.