@SteppinRazor my thought has been to bring them or F4 in through dimensional travel. It allows us to avoid another origin while also explaining why they weren't there for Thanos.
I still wish they found a way to put Chris Evans back in the flame suit or show any xman when they were playing with time in Endgame.
Think it'd just be a nice gesture to the actors and a nod to the fans to have Jackman or Stewart show up even for an instant.