This "balance" of The Avengers is IMO nothing that the X-Men movies should try to copy. It was so busy hopping from one character to another to another, it just made the movie feel like it was full of supporting characters with no real development to anyone.
If you tried to be "fair" to all the characters in X-Men movies, you wouldn't end up with a movie where every character is a Wolverine, you'd just have a movie where every character is a Banshee.
It's not a question of copying Avengers.
Avengers proved that a more even spread of character focus could work (and it also proved heroes with individual and colourful costumes could work on screen as a team).
What the X-Men could do is to shift the focus off Wolverine a little and give a bit more substance to some of the other characters. The same thing happened in First Class - it was all about Magneto and Xavier to such an extent that other characters (Banshee, Havok, etc) felt like filller who were just there to get X-Men fans to see the movie.
No one's asking for actors/characters to have a forced role like Halle/Storm in X3, that's going to the other extreme.
But all characters should have a good reason to be there - a motivation (indicated in dialogue, interaction and action). And it should be tied to the main plot, so that way it keeps the focus.
Just off the top of my head, and as an example, let's look at Storm and DoFP. If she is included, she shouldn't just be there for special effects. What's her purpose, why is she fighting?
We know she was headmistress of the school in X3 and has always had a 'mother goddess' aspect (spinning off from her elemental powers and their connection to the Earth and nature). It also fits in with her role as a protector of the tribes when back in her native Africa, where she was called on to bring rain for the crops and livestock.
In the films, that 'earth mother' aspect was shown in her being a teacher and in being concerned for the children's welfare (when at the dam in X2). It wasn't developed enough, but it's something.
So, if she were to be in DoFP, that angle could be expanded upon if that's the characterisation they choose. She could be the protector - of the surviving students, or of the young mutants at the prison camp, or whatever. They just need to make it tie into the main plot, so she feels an integral part of the story.