"Who?" is what most people would say.
Even though it's Fox's movie, Deadpool was a massive hit at the box office and is easily one of the most popular Marvel characters in years. Not to mention Disney/Marvel make A LOT of money just from Deadpool merchandise (this isn't including shared profits from the film's merchandise). Same goes for X-Men, but not nearly as much as Deadpool. He was already a pretty popular character just from word of mouth for years, but nowadays he's on his way to be along side poster boys like Spider-Man, Iron Man, Captain America, etc.
People can argue all they want about the mobile games and how X-Men characters are absent, but this is Marvel Vs. Capcom. The fan base for this franchise is much more vocal than any mobile game community and much more passionate. If Marvel, Sony, and Disney could work something out for Spider-Man then I'm sure something will (or already has) for X-Men. Having Deadpool (or any X-Men character) would be a win win for literally everyone (fans and from a business perspective).
Also, I know some people will point at Lego Marvel Avengers, but that released before the "Deadpool boom". If I'm wrong then I'm wrong, but I don't think these big companies are that dumb.