Why not do both? X4/First Class... Combine The Franchises!
I actually had a similar concept in mind... but, instead of involving time travel, my idea was to just have characters of the first class team return as old men/women, to warn and seek the help of the current x-men, in the return of the old enemy that their team faced back in the day... whose is now tracking down all the former members, that defeated him/her/it all those years ago...
and they would like kinda go back an further in the timeline, telling the original story in flashbacks of what happened little by little throughout, and then back to present times of what’s happening, now (and how the evens of the past have effect the future)