The thing there, is that even after dropping the turd that was Other M, Sakamoto likely still has the juice to veto or sabotage any efforts for Retro to officially take over Metroid.
If that doesn't happen, I hope it's Metroid Prime 4. Or, Metroid 5. Enough with that prequel, side story garbage, let's get on with the narrative, as after Fusion the series could have actual potential for story telling and character development unlike that shoehorned baby baby baby baby crap.
A 2d Mario game would be neat. Perhaps Retro could make Mario cool again, give him back some grit and personality. NSMBWii and DS were fine games, but they really stripped down much of the character of the Mario world. There's all ages and approachable material, and there is bland and boring. Mario has fallen far into the latter.
Like Mickey Mouse, the mainline 2d Mario games have scrubbed up and down and sanitized to their modern, bland state. It would be fun to see something like the wild and inventive settings and stages of Mario 3 placed back into 2d Mario.