3D. I'm not totally sure what could be done in a 2D Metroid-series game that we haven't seen before. That's not to say that 2D Metroidvanias don't have anywhere to grow--I just don't see a theoretical Metroid Dread being super-innovative. I feel like it would be yet another Super Metroid-esque game... which could be great, but the possibilities for a 3D Metroid game to tread new ground seem much more easily apparent to me.
I really think Metroid is due for another 2D stroller game but with similar 3D visuals used in Donkey Kong and Mario Bros. We haven't had one since the GBA on the other hand we had 3 3D game come out since Prime 1. Either way I (as a not huge Metroid fan) would still love to see a new Metriod game.
@Hinph Yeah ideally minimal is best, I really like the cut-scenes in Super Metroid and Zero Mission, Fusion was maybe too chatty at times, but it made sense with how Adam (freaky AI ghost version) was a main character ala Cortana from Halo.
Just so long as it isn't being handled by Team Ninja, I couldn't care less what form it takes. The Prime games were amazing, but the formula became tiresome quickly - with all of the scanning (yes, I know it was optional) and the constant intense battles (where the doors would seal, the music would kick in, and hard-to-kill bad guys would drop in). I want Super Metroid Prime. The best of both worlds. More exploration, less forced action.