It's a 2.5d Metroid game for the 3DS, just announced. Yes, really. Apparently it's being made by Mercury Steam, overseen by Sakamoto, so who knows if it'll be good or not but this sure is a nice surprise! They're showing gameplay now and it looks fun.
And... it's a re-imagining of Metroid 2 for the Game Boy. Yes, really. AM2R or this, which will be the better Metroid 2 remake?
You can drop pins on the map to mark important spots! That's a FANTASTIC addition to Metroidvania map design. Great idea.
Interesting, and that's a good question. Clearly, Nintendo has at least heard the concerns about whether the Metroid series died with M (Federation Force is a sub-par multiplayer game with a Metroid skin, so it counts about as much as Metroid Pinball does).
I've seen people saying that this probably explains why Nintendo went after AM2R like they did, and it makes sense when Nintendo is making its own remake...
As for the two games though they may both be based on the same game, but they're pretty different since this one is trying new things, as opposed to sticking to the 2d Metroid formula. It has 2.5d graphics of course (with 3d support), but gameplay-wise you can aim in all directions in this one instead of only left/right/up, and that changes things significantly. I'm sure there will be more beyond that too.
The press who have seen this game like it, apparently, so I think there is good reason to be optimistic even though MercurySteam doesn't exactly have the best track record. Nintendo usually gets the most out of developers who work with them...
(Oh, and while Nintendo made some great announcements at this E3... no F-Zero, again. Why??)
AM2R did new stuff as well. It was a thorough remake, implementing various features from across the franchise as well as brand new ideas. You should check it out. And yes, it did let you aim in 8 directions instead of just 4, like every Metroid since Super.
This one does seem to be going a bit further, since it looks like it'll have analog control that'll let you aim in every direction instead of just 8.