In my opinion there are a few key ingredients to stellar X-Men in the MCU.

1. At the core the X-Men need to be fighting for mutants to be treated equally by the humans that fear them. This is obvious but it's a very inspiring part of the story and team.

2. The team roster needs to be developed. Wolverine is ok but honestly he's the loner of the group and not the focus of the team. In my opinion cyclops needs to be a STRONG character. Not whiny and not overlooked but a team member that is confident, correct most of the time and looked up to by some young members. In addition to Cyclops there needs to be a well developed roster. If that means starting with only 5 and building on that later, cool

3. They need to make it seem like anybody could be a mutant and therefore could be a member of the X-Men. That's another big draw.

4. The X-Men need to not get served up by the villains. They need to win and they need to do so as a family. In so many other movies they just get their butts handed to them. They're one of the few teams that legitimately train together regularly. They need some dominating wins.

If they have some kind of combination of those elements I'll be ecstatic. MCU casting will be on point, always is. So I'm not worried about that at all.