Re: X-Men: Part 6 should be set...
an easy option?:
To introduce Sinister on the 80's.
And use him again on X4 with the idea of him serving someone bigger than him (Apocalypse) or even to go full out with both as lead villains on X4.
Sinister doesnt age right? so there you have it.
He uses young Jean and Scott on the 80's, and back to present, he tries it a second time. Perfect arc for him on the franchise.