Our girl Emily Rogers claims that Retro isn't working on Metroid, or "Fucking Donkey Kong" for that matter.
Granted, she isn't always 100% correct (see the Female Link thing) but it's still something to consider.
Update: Thanks Emily.
About that *old* Retro Studios rumor: it came from a second hand source (game dev from Austin, TX). New IP could mean different Nintendo IP.
Hopefully we get an original Retro IP.