Prime 2 has me more divided and conflicted than any other game possibly ever. There are parts about it I love, but there are also huge things I can't stand. I liked Prime 1 more for the same reason a lot of people didn't like it as much: It was 3D Super, basically.
If I could change one thing about Prime 2, it wouldn't even be the darkworld. It would be removing beam ammo, and allowing you to get the Sky Temple keys throughout the game, and not all at the end (like the Chozo artifacts; you need all 12, but you can get most of them at any point along your journey). Okay, that's two things but yeah. I wouldn't mind it if they made it a difficulty setting. Call it "whiny bitches who hate beam ammo in mah Metroid" mode.
Edit: One thing I will say I'm excited about is the potential for the controls to not be ass. We may get GameCube like controls. I loved the first two's controls, even though they aren't traditional two-stick shooter controls. They could also offer a more traditional control scheme since the Switch does have two real, competent sticks. And, they can even satisfy those who liked motion controls, since hey, two joy-cons. I'd be more inclined to use motion controls if I knew a button alt. was right there.