With Sakamoto at the helm there's not a lot of hope for this game. Even if he doesn't Other M it, the best this guy can do when he's not supervised by Gunpei Yokoi (Rip in peace) is to make a disappointing game that leaves a bad taste in the mouth. Sakamoto was so butthurt over people liking Prime more than Fusion that he tried to write Prime out of canon altogether. Now, I can't help but feel he's trying to tear it down from the inside by making an entirely uneccesary sequel to a sub-series that had a solid and final conclusion. The Metroid Prime was completely destroyed at the end of Prime 3. So why on Earth are we getting a Prime 4?
There's not really anything I want from Prime 4. I'm not a fan of anything continuing past its logical and reasonable ending. Metroid Prime is over, we did it, we won. Don't drag it back up for no reason other than brand recognition. Anything that happens in this in terms of story will be one of two things: (1) contrived nonsense that undoes the satisfying conclusion of the previous Prime games, or (2) something completely unrelated to the other Prime games which fails to justify the use of the Prime brand. I don't want either of those things.
In terms of gameplay, I mean, sure I'd enjoy playing a game almost identical to the existing Prime games. Some tweaks and additions wouldn't be unwelcome as long as the fundamentals were in place because Prime's core gameplay was great. Really the only thing I'd say they needed in terms of gameplay would be more space. Just give us more space to explore and find secrets in. I'm sure I'd have fun with such a thing, but I don't feel that would justify reviving a completed sub-series. Change the gameplay too much and how can you call it Prime? Change it too little and why revisit the brand at all? Again, either way, it's not something I want.
As for what I expect, well... Sakamoto proved with Other M that he has no idea how to handle Metroid in a 3D environment and that he is the worst writer this side of FanFiction.net. In terms of story and gameplay this dude is hopelessly beyond his depth. I'd offer vague hope in the notion that Bandai Namco aren't incompetent, but I would have said the same about Team Ninja eight years ago and look how that turned out. I feel bad for being so cynical and down on a game I know a buttload of people are excited for and interested in, but I have absolutely no faith in the man making this game, and its very conception is something I disagree with on a fundamental level.
