I don't mean to say the Borg are currently overpowered in terms of being Borg, but rather in terms of being a remotely playable race in a fair game. The goal should be to give them something unique and distinctly Borg, but without making them more or less powerful than, say, the Romulans. Transwarp conduits may not be the way, but I think it's a step in the right direction.... Actually, I have an idea in that direction. Why not give the Borg their single most feared ability? Assimilation. This could be done in a number of ways. The first I see is by simply reducing the time it takes for captured bases to convert from their original race to Borg. The second is by using tech stealing: the Borg, and Borg alone, would be able to steal an enemy's technology after stealing an enemy's base. The third is enslavement: a defeated enemy ship would have a change of being captured rather than destroyed. Maybe even all three. Assimilation would make the Collective a force to be feared in war without making them unbeatable in combat.