Some people will need a lot of time in the gym to do it.
Because in the future everyone looks fabulous "25 years later".
How would they go back and fill in the gaps with the same actors?
1. Well, yes, on the gym issue.
2. In the Endgame episode, they used make-up and gray hair to age the characters appropriately. It's now 11 years since the show ended, so the actors aren't actually 25 years older themselves at this point. So depending on where they start filling in the gaps (how long after Voyager's return), they may not have to "youngify" the actors all that many years.
I do however agree with whoever said there wouldn't be enough action with this setup. This is why I could never be a writer - I unfortunately have no talent for plotting interesting storylines.