I really think it would be great if we had mini-bosses like Tortodrones. There doesn't need to be lead in levels or tokens, just big scary things to spice up regular levels. Maybe they should only occur in T3. Honestly, to make me happy you wouldn't even have to create new enemies, just reintroduce Tortodrones.
I believe they removed it for a reason. Maybe the design was too old, to even be considered as something they would keep in this game. I for one, would prefer the tortodrone to be redesigned though, and then re-added to the game again. Always awesome with a mini boss C: !!