I think that would be a very bad idea on two counts; firstly, realism: this isn't "Field of Dreams." In the real world, people don't go and watch rugby just because it's a big stadium, they go because of tradition, because their team is successful, and because it play a good rugby. Edinburgh playing at Murrayfield is a classic case in point.
But more importantly, it would encourage even more people to build stadia far bigger than their teams would justify, which would lead to even more GvG mismatches.
Ideally, fan numbers should be based on league and ranking positions, taking ticket prices into account to make sense and keep the GvGs more even, but the current system would be ok if people worried more about team quality and strats and less about fans - who would actually want to see the worst team in the world?