I see two types of themes that tend to be weeded out... the really bad ones, and the really good ones.

The best themes seem too limiting at first glance, and since there's so many to slaughter, they never get that second glance.

In the final rounds, people will vote for their favorite, but also the ones that aren't absolutely terrible. The former being more polarizing than the latter means that the "eh it's okay" themes wind up being the winner.

I think it'd be better with we only were allowed ONE suggestion. If we had to really put thought into our one suggestion, we're more likely to make it a good one. It'd make the initial slaughter round less time-consuming and give us a better selection in the end.

Like, consider this. What if in the end there are three choices and only two people voting on it, and they have to pick there first and second choice: 1 - Great theme that the first person loves but the second person does not. 2 - Great theme that the second person loves but the first person does not. 3 - Generic and mediocre theme the both people feel they can work with.

In the end, the first theme gets one vote, the second gets one vote, and the third gets two. The generic and mediocre theme wins out.

Scale that up to ten themes and thousands of voters, and the great themes have no chance. The throwaway themes that people suggested just because they felt they needed to have three win every time.

On the contrary, the greatness of the other themes will become evident to the person who didn't choose them once they actually start developing.