Back in the NES days... or even SNES Metroid was about as grown-up as you could get. The sci-fi, the bleakness the shooting, the creepy aliens. More adult themed titles are not totally foreign to Nintendo. Metroid is still somewhat like that, but unforunately the theme is a bit niche. It's like Aliens is for film... the name is odd and geeky, it's got a female heroine (who doesn't even look female which is double no-no) and the game is too methodical for most.
Nintendo could really use a game like Halo was when it launched. Cool, fresh, adult but not overly violent, long and deep but still action focused and mass-market friendly. Metroid could be that title but only with a major overhaul. The name would need to be changed (it could still be in the same universe, just not focused on the life-sucking aliens) Samus should lose the old outfit and get something more sleak and sexy (the bulky shoulderpads were only added because the gameboy couldn't show color anyway), the pace quickened slightly and maybe a second player co-op added with a cool new partner. A bad-ass alien or a male bounty-hunter with cool new abilities.
That said, I would prefer them just making a new IP. Like it was suggested, Retro is a great fit. They've recruited great talent from Naughty Dog and others. Make something fresh, action-packed and cinematic. It would complement the rest of the lineup so well. Metroid could continue being a niche thoughtful adventure game and this new series could make the WiiU fresh and cool. It would be great to have this and old favorites like Mario and Zelda. I have my fingers crossed that something like this is Retro's unannounced project, but I think the title will be something based on an existing IP.
