I tried pretty hard in my original proposal and subsequent comments to fairly address what I think are two biggest concerns:
1. It's too dangerous to give players the power to control a Borg cube.
2. Playing as Borg means PvP, and non-consensual PvP is unacceptable.
Both these concerns are absolutely worthwhile. If a Borgplay feature doesn't address them, it shouldn't be implemented.
That said, I think they have been addressed:
1. No individual player has control; what a cube does is the average of what every drone (including NPC drones) wants.
2. Borgplay could be restricted to its own special sector, complete with plenty of things to see and do, and the only way to access it is to explicitly opt in.
I do disagree with the theory that the Borg-zone approach would relegate this kind of play to a niche feature. Of all the ideas ever seen in any Star Trek show or film, there is no question that the idea of the Borg has generated the most excitement in fans. To be able to play as -- or fight -- the Borg in ST:O could, IMO, turn out to be one of the most popular parts of the game.