It would be even more fun if it was the Fox cast. Snagging Jackman, Stewart and McKellen would be a big get. And because this is only a “What If…?”, they don’t have to worry about establishing an MCU proper mutant story. Worth noting, like with Raimi’s Spider-Man films, Kevin Feige was also producer of the initial X-Men trilogy, just prior to running Marvel Studios. So there’s already a history there.