How about instead of new characters, you unlock a set of reskins for the existing characters? Like, if I beat the Secret Act for the first Zone, I unlock the option to still play as Sonic with no new abilities, but now he's either using Modern-style graphics like Sonic Advance, or 8-bit Master System/Game Gear/Pocket Adventure graphics. That way, each secret character can have optional unlockable skins as well. Hell, if you set up the tilesets correctly and have a single option that loads a different tileset that's organized identically, you could even go so far as to have reskins for entire Zones or Acts. Imagine getting to play Time Trials of the first Zone, but you're playing with an 8-bit Sonic and the Zone's been reskinned to look like it's from the Master System. The Zone's and character is still identical, but it looks completely different. Of course, going that far, if there isn't an easy way to swap the tileset, would be way too much work. Manually doing it would end up being like building the whole game all over again. If you could pull it off, it'd be like playing Super Mario Bros. Crossover, only in a Sonic engine.