You don't keep doing a movie with the same teams and same characters and say you're expanding a universe, that the complete opposite. Expanding the universe means following new or different characters in new storylines and situations. It needs to feel like there is something going on outside of the main X-Men team.
Uh, what do you call Guardians of the Galaxy? That certainly would qualify as a spin-off or "expanding the universe." Not to mention there's talk of Black Panther, Ant-Man, Doctor Strange, The Runaways, and Inhumans movies.
That's universe expanding and world building. If Fox wants their X-Men and mutant franchise to keep up and compete they aren't going to have to think about doing something like this, they are going to HAVE to do this.
And I didn't say anything about adding new members of the X-Men and introducing new villains? Read some of my posts where I wrote that prof X/Magneto/Mystique/Brotherhood shouldn't comeback for the next movie. And they don't have to release spin-offs just to expand their universe. They just need to introduce new characters and maybe go to new locations and of course they'll come upwith a new story for every new movie.
And Guardians of the Galaxy is definitely not a spin-off. Its a brand new film or series, and most viewers won't see it as an Iron Man film, Avengers film, Thor film and Captain America films even some of the characters might appear in the next The Avengers movie. With X-Men, it doesn't work out like that, release a First Class movie, release a Wolverine movie, release a X-Factor movie, release a New Mutants and I'm pretty sure FOX would let the viewers know its under the X-Men films category or its a X-Men spin-off/prequel film. With MCU, even if they have those cameos from Iron Man, Nick Fury, people will not see them as spin-offs, people see them as their own thing. Well unless they release a solo movie for Winter Soldier, Loki, War Machine, Black Widow, Hawkeye, Nick Fury, Rocket Raccoon, they will definitely be seen as spin-offs and as of right now, they have no plans in developing a solo film for those characters.
And even if MCU releases film every year, you won't hear the mainstream audience saying "another MCU film?" because most of their characters can stand on their own and they won't have to rely on brand/popularity/name of Iron Man, Thor, Captain America, Hulk... they can be seen as their own thing. But with the X-Men, I expect people to say "another X-Men film? another X-Men spin-off? when are they gonna do X-Men 4 or 5" if they release a new X-Men-related film every year.
Thats why FOX should just focus on making movies for the original cast of the X-Men and Fantastic Four. Even if the next FF film flops it won't have an effect on future X-Men films. But if a Wolverine movie disappoints, a First Class movie disappoints, it could harm the future X-Men movies.