1. Robotnik shouldn’t return
Why? Because his ending in the last movie was genuinely heartbreaking and emotional. The way he called Agent Stone a “sickle friend” and Stone’s reactions, as well as Robotnik’s “If I can’t rule the world, I might as well save it” line was genuinely a perfect and emotional send off for the character. And I feel like bringing him back would just ruin all of that. Hugh Jackman may have came back as Wolverine after Logan, but at least they found a good way to bring him back without completely retconning Logan. I can’t see them doing the same with Jim Carrey as Robotnik (as I doubt we’ll be seeing any Alternate Universes in the movies). I feel it’d be best if they just kept Robotnik dead.
2. Sonic 4 should be the finale
As I had said before, bringing back Robotnik would just ruin everything that made his sendoff in 3 the perfect sendoff. And I can’t see them making more movies without Robotnik. One movie without Robotnik is one thing, but a bunch without him, it just wouldn’t be the same. Even if they do bring him back, I still think 4 should be the end because I’d like to see the movies end on a high note rather than milking the series just to make a quick buck. Honestly, if you ask me, the Third Movie would’ve been the perfect finale, but then they introduced Amy in the Post-Credits scene. Besides, I don’t know if they’ll have many ideas left for more movies after the fourth one.