Interesting idea
The big flaw would be if a couple of people from the same alliance, especially one hated by the viewers, finishing in the top in the challenge and we ended up with a quite a few weeks with only members from one alliance nominated, like the Luke vs Lauren head-to-head that annoyed so many people last year.
I'd like the idea of just having the winner of the task be immune and the first person out end up automatically up for eviction, a bit like 'I Love Money'. That way you'd get three public up for eviction, as a minimum every week.