Koei Tecmo and Nintendo have quite a good relationship with one another, thanks to the Hyrule Warriors/Fire Emblem Warriors collaborations. Koei Tecmo’s president Hisashi Koinuma would love to work with Nintendo again in the future, and he's eyeing up Nintendo's most value IP.
“I’d like to have an opportunity to work on Nintendo’s Mario. Basically, he’s a character I’ve been playing with ever since I was a kid, so if I could do something with him, I’d be really happy. I think anything would be great, even a simulation would be good.”
I'm not sure that a Mario Warriors game would make much sense, but then again, what do I know?! I wonder if Koei Tecmo will ever get the keys to the Mario kingdom. Ubisoft got their shot and it turned out pretty damn well!