I wonder how many people thought that a huge crossover of several Disney franchises and Final Fantasy would work back in 2001. This crazy combo did, though. And so well that it has spawned enough sequels to become a franchise itself. I am talking about Kingdom Hearts, of course.
Provided this rumor is true, I will reserve judgement until I know more about the game. As a long time Star Fox fan I do find the idea scary, but I can imagine that a scenario exists where it would work, and if anyone can pull this crazy idea off, it's Retro.
But I am concerned. My biggest problem with Star Fox Adventures was the obvious feeling that you weren't playing a real Star Fox game (which it wasn't, it was Rare's Dinosaur Planet re-badged with Star Fox shoehorned into it). But, given the difference between Star Fox and Metroid, I don't think it is possible to retain the feel of either series.
But, I think Star Fox has more to give than Metroid here. Many Star Fox fans have wanted to see the series get a little bet "darker" than it has. While this certainly isn't the totality, there are enough fans out there that want a darker game that I don't see it being an issue. This crossover could see how the Star Fox characters work in a darker environment.
As far as Star Fox gameplay, On-Rails can only do so much these days. Most fans I've dealt with wouldn't mind seeing more of a Star Fox Assault experience, but actually done more solidly and longer. The game felt unpolished. Polished, it really could have been that "true sequel" to Star Fox 64 we've been wanting.
If you take the Star Fox Assault model, there is a way you can create a unified experience between Star Fox and Metroid. The game would bring a bit more on-foot play to Star Fox, while bringing a bit more vehicular combat to Metroid.
I never got seriously into Metorid, so I can't really provide much in the way of analysis from that perspective. But from the Star Fox side, it can work. Maybe. But the potential for a disaster is high. This kind of crossover needs to be approached very carefully. Who knows, the idea may just be crazy enough to work.