Metroid.

After seeing how Nintendo seems to be returning to making big, ambitious titles like Breath of the Wild & Mario Odyssey along with inventive new IP from core genres like Splatoon & ARMS, it would be really disappointing to see Retro so what you described.

I've heard that Retro expanded are working on two games now, but I'm not sure if that rumor is true or not.

If it is perhaps they are working on a new Metroid Prime game featuring Samus and Sylux as well as maybe a new IP. It's hard to predict what genre it will be since Retro is so vast and talented that it could be anything. There really haven't been any hints to suggest a single genre. All I hope is for the game to be great both in terms of gameplay and visually. Really letting Retro shine bright once more. They were, or are one of the most talented western developers up there with Naughty Dog and BioWare. The question is.... are they still that good or with staff change will they go the way of Rare? It's really an interesting time and I'm very excited to see where Retro goes and what Nintendo will do next. I'm expecting some big surprises this E3 :)

Yeah, I'm really hoping it's something big and bold. Ideally I'd like it to be a new IP along the lines of something like Uncharted or Bioshock, but I realize that's unlikely. At any rate, if this E3 comes and goes and we still don't know what they're making, I'm gonna go apeshit. The wait to see what they're doing next is killing me! XD

I can't tell what they've being doing but now would be commercially the right time for Nintendo to release a new Metroid. So I have a little hope left.

I'm thinking a new IP, but possibly Prime with their 'teasers'

Descent sounds edgier though. More appropriate for the mature GAMER.

...frick though, Ascent sounds way better...  

Edit: Dang it I quoted myself. Too lazy to find Tak's post again, but this is for him.

Descent for sure, a group of mountain climbers just successfully climbed the worlds tallest mountain but on the descent strange things start to happen and one by one the climbers begin to mysteriously vanish and a bad storm is approaching. Will any of them make it out alive?

It just isn't E3 season without Startropucs speculation/rumors...

If Nintendo truly hasn't learned anything (meaning they want an over abundance of platformers at the cost of gameplay diversity), then I think we can anticipate a new 2D Donkey Kong...