Eternity in an hour
People still love Sonic 2 and CD despite being two games that don't have a secret boss when having every Chaos Emeralds/Time Stones
Sonic 2, yes. CD on the other hand is controversial. I happen to enjoy it myself, but there's a lot of people who think it's terrible. The only thing I think is more or less universally enjoyed about it is the soundtrack, and even then there are arguments as to which of the two OST's is better.
Back on topic, I'm also in the boat that I would just be happy to see a special Super Sonic boss fight at all, though I would prefer it to be 3D in some capacity. Perhaps it could be (in regards to level design) designed specifically around Super Sonic's moveset without changing up the moveset from regular gameplay, so as to make it interesting yet familiar. NiGHTS mode would work, but it would also be a tad underwhelming since the game is primarily 3D.