Instead of licensing those old IPs, if Nintendo really wanted to they could have Retro make clones of them. Like Battle Toads, make a game called War Frogs, or something and just change the names and the various details to avoid lawsuits, but have it be blatantly the same thing in every legally possible way. And the same could be done with Killer Instinct by creating a game with a very similar name like Lethal Instinct or something, and have very similar characters and very similar moves and so on. Make it different enough so that lawsuits are avoided, but keep it as close to the old games as possible.