I was thinking about that today during spanish (since we are doing basically nothing in that class right now) and decided that characters who don't age, like Akaelin, Nathifa, Jetta, Runi etc, would still exist, but we just might not see them. They could have a role somehow, but we could tie them in without having to explain much. As for who ends up with who and who dies, this doesn't have to be quite straight with the story. If we kill off someone here that would have a descendent in the branch then we can always say "whoops, they were actually descended from this relative of the dead person" As for who ended up together we can just improv as we go kind of doing the alternate reality thing
It'll be like X-men! I can't tell you how many alternate realities are in that comic...too many to count.
Kitty: I haven't died!
Cyclops: Yes you have, you've died twice!
Lockheed: Scraaaawwwk!
Kitty: Lockheed wants to know if we're counting alternate timelines?
XD That was from a parody on youtube