Well, they will probably use 1 ou 2 more mutants for the past although i think that less known mutants. And i agree that it wont be just 1 or 2 mutants traveling to the 70's. Time travel will include more from the triology. The story will focus in the 70s period so we have to have more mutant interaction there.
And i am sad that Marvel will use Magneto children. I like marvel movies, but i prefer the mutants universe. And id love to see some interation between Magneto and Scarlet Witch and Quicksilver and Polaris. That would be great.
I think the next movie could focus on Genosha story, and bringing Magneto children at that time would be the ideal move!
But dont know if it will ever happen