(Wildly speculative) idea about the next overarching threat - *spoilers*

[Iscah]

Wasn't a big thing about 8th rejoining that if it happens, all remaining shards also will naturally rejoin because balance of 'sundered' vs 'rejoined' shards would tip too hard?

There was never anything remotely like that. It might have been someone's theory as we tried to make sense of the very-pre-Shadowbringers "eighth and final Calamity" statement, or however exactly they worded it.