Pretty nice summary you did there! It should have been in the first post
Although I have to disagree with you about the themes you suggested. -Its not that they wrong, they just doesn't suit me. I would use the least imports in my race, so it would be hard to make a mythology race -a space race even more- because i'd have to search and import models. Don't get me wrong, its just my point of view.
The environmental race idea is what i like, but i wrote it down before in the thread.
Optionally paired: Really interesting idea, making the details would be much more easier for contestants, and for that, there would be lot prettier entries than what we have now. Pairs may finish in time, fix the bugs more effectively. But if the contest is paired, lonely ppl would face unfair disadvantage. - Solution for this is make finding pairs easier!
(Also i see a chance for the 2 custom races idea here)
continuing in the second post