I have an idea, they don't bring in the existing FF and x-men characters we've seen, they create new one's. And they do this by the following:-
Rocket says 'when Thanos snapped his fingers, earth became ground zero for a power surge of ridiculously cosmic proportions, no one's ever seen anything like it'.
If you go forward 5 years to the final, the snappening happens twice in the space of maybe a few hours, once to bring everyone back, then again to wipe out Thanos and his armies.
So three times in 5 years, that's a lot of cosmic power, and we know that power radiates gamma, Hulk told us, so maybe mutations start to develop as a result of all that gamma radiation handing around earth and hence, a new set of x-men and FF are born/formed/developed. Just an idea......