Someone is definitely working on something right now. This is, after all, one of their strongest core franchises. However, it's really hard to say what developer is going to be the next to take a crack at it, and what form the game is going to have. The problem is that if it isn't Retro, then the FPS route is likely out of the question. In theory, I could envision Metroid as an open world third person shooter with clever platforming and puzzles.
As to when we'll probably hear about it - so far we know of Nintendo's exclusive software into the first half of 2014. Smash Bros., Mario Kart 8, Bayonetta 2, X and Fire Emblem X Shin Megami Tensei are all likely to come out before fall (I'm personally feeling like Mario Kart and Bayonetta are coming out in late winter to early spring and the rest around May-September).
That leaves the holiday season for 2014 empty so far. However, we do know that Nintendo is definitely working on a new Zelda for the WiiU (which apparently was almost ready to be shown at this year's E3). Furthermore, with the timing on the AC Plaza Channel, it is definitely possible we'll be seeing an AC for WiiU around December 2014. Also, there is still the Epic Yarn - style Yoshi game in development for the WiiU. In addition to that, Myamoto is currently working on a new IP which is at least conceptionally finished by now. Still, that would leave the end of 2014 fairly empty, which then begs the question: what else does the big N have in store for holiday 2014? They certainly have enough existing IPs to work with, and one should always assume that there are 1-2 projects that nobody has even the slightest idea of.
All that information combined leaves the optimistic impression that we might be looking at late 2014 to early 2015 for the next Metroid game.
