Let us assume, by some miracle of copyright negotiation, a special Doctor Who/Star Trek TV movie was produced. How would you feel about a storyline that shows the Borg and Cybermen to be one and the same?
I think it would be really cool. Both characters function on basically the same premise: an emotionless race that augments itself to achieve perfection, and thinks everybody else should join them. The correlation wouldn't take anything away from either franchise, and would actually add some interesting layers to both characters of both shows. If done properly it could be a lot of fun!