I think that Retro Studios's competences are wrongly estimated around here.
They have worked on Donkey Kong games plus helping Nintendo's other projects in the past 6 years. Part of the developers who worked on Metroid left long time ago. Their only HD game is a 2d platformer. I don't understand how under these circumstances and without having a big restructuring of the studio one could expect some huge projects from them like an AAA open world or something?
The Metroid Prime talents can be replaced. That said, I don't expect it to be a AAA open world game.
I just don't know what it is.
On a sidenote, I don't find it being AAA or not particularly interesting. I don't play marketing budgets.