Re: wat if they built up to x men, like avengers?
That "what if" you are proposing wouldn't work.
Because unlike Iron Man, Hulk, Captain America and Thor, they could always be introduced as their own thing first and during those years, Avengers wasn't exactly popular yet.
With the X-Men, they are already known as a group of mutants with special abilities in the 90s. So when a studio/company releases a X-Men movie or a series, people would be expecting to see a couple of mutants, not an origin story for just 1 mutant that might become a X-Man in the future.