Discovery of some sort of warp in space-time "wormholes" or the like to jump the distances. Otherwise the relativistic time issues are going to create a bunch of totally disconnected nodes of humanity never to contact each other again. Once someone has left, that is it. No further contact, ever. in a useful manner. Radio conversations with multiyear pauses between replies do not work so well I should think. And that would be for close neighbors.
Another possibility is that we find out how to turn the genetic trigger for old age off or the one for indefinite long life on. So if there aren’t any insurmountable mental problems we could perhaps operate on huge continuous time scales without having to rely on future generations to continue our really long term projects.
Freegards
It already exists. Bob Lazaar worked on them at Area 51.
(I believe him, seriously)