In terms of a franchise crossover, they could do it so whoever from the TNG crew (& possibly DS9/Voyager characters) appears are from this timeline's future. And at least one ENT person and his dog has already been referenced. Admiral Archer just turns out like Cooper from Interstellar, who went through some weird time warp, while everyone on Earth got old or died. Nimoy's already there. That just leaves a Shatner surprise.