Borg is impossible without completely ruining their background. There are no individuals, no captains aboard Borg ships, no social hubs, drones don't even speak to eachother. And their ships would have to be balanced against Federation/Klingon ships and thus be severely underpowered compared to canon.
They are much better suited for a PVE endgame faction imo.
First playable faction should be Romulans, as they are part of the original big 3 powerful factions from the original series. Then the Cardassians, possibly as part of the Dominion.
lol ballance? Klingon > Feds there is no ballance at all. Why shall they ballance Borg?