The Prime series is over it was only supposed to be a trilogy and honestly that is a good thing considering how absolutely aweful 3 was.
I think they will go back to a 3D Metroid model considering how bad other M bombed for them, the prime series sold much better.
If you do see another entry in the 3D metroid games expect to see it on Wiiu, they basically closed up shop on the wii and are focused on making games for that system now.
That being said Metroid being on a HD system is going to have the expectations placed on it that a AAA HD game has, the current type of game Prime is won't even cut it, there will have to be better combat, smarter A.I. and above all else it will have to control more like a 3rd/1st person shooter in modern games.
That is if they want to sell it to gamers who aren't sold on Nintendo franchises.