###They'd probably just wait around...
They'd probably just wait around...
It's surprisingly tricky to find anything online pertaining to the Borg's long term goals post galactic assimilation, but its reasonable to assume that as the Borg ignore lesser civilisations which offer no technological advancements, eventually they'll begin to ignore more and more of the galaxy until they have no other option than to just wait around for someone worth assimilating to come along. They'd probably become the most accomplished explorers in the galaxy, they've already travelled to fluidic space.
I could even imagine they may begin to encourage different species to evolve and develop in order to assimilate them when they had something to offer.