As I was washing my hot ass in the shower earlier today, I had the most glorious vision of what this rumored Star Fox x Metroid could be like. First, imagine the Metroid Prime bits to have dynamic lighting like the Dead Space games. You know, shifty shadows and all that shit. Now picture the Star Fox bits like Rogue Leader if we'd gotten one this gen. A whizbang Arwing mission here, then cut to first-person exploration as Samus; gain some power-ups, backtrack a bit, beat a boss and get prompted to cut back to Fox or continue as Samus. Keep at it until both sides complete the first half of the game, when they meet, and Samus takes to a ship and Fox hits the ground exploring. Bam!
Now rationally, I still don't believe this rumor, but the more I think about it, it does seem like something Nintendo should do, purists be damned. Aren't the Japanese kinda "meh" on each franchise individually? But they sure love them crossovers, don't they? Layton and Phoenix Wright, Pokemon and Nobunaga's Ambition, this Sega/Capcom/Namco crossover, etc. Put Star Fox and Metroid together and suddenly it' an event, something epic you can't miss.
Plus, from a sheer "how to" standpoint, Retro would essentially be making a Metroid Prime, which they already have great experience doing, and intercutting it with a Star Fox game, which being an arcade shooter is formulaic enough to make. Not saying it's something to sneeze at but the thought process is clear enough. The zippiness of one supplements the thoughtfulness of the other. Would be grand and a graphical showcase set in outer space, no doubt. Alongside Pikmin 3 and, gods of gaming be willing, a new F-Zero, it would be like all of Ninty's bounty hunters are mustered for WiiU.