I feel like they desperately need to introduce some kind of new long-term plot hook, because I don't think making a transition to completely self-contained stories for future expansions is going work out well given all the post-patch story that has to be considered.
The only thing that really springs to mind is the state of "natural" rejoining hypothesized by Y'shtola resuming now that Hydaelyn is gone and we have to deal with the unpredictable consequences of such over the next future expansions before ultimately finding a way to stop it before the shards and the Source as we know are toast.