I'll write what I'm planning, that's it. Personally I've done well with fairly long pieces, so I'm not very enthusiastic about too many time constraints. Also, there is already an inherent difficulty on writing a multi-movement piece and have it thoroughly reviewed by the community (which is the added benefit of these competitions), so if we go once again into requesting miniatures, we'll never get past that.
In that case, we could also argue that anyone writing a choral or orchestral work would have an advantage as well over chamber/solo works, due to the supposed bias towards longer, larger, more complex works (a bias I haven't seen any real evidence of). So that might pull the trigger on a request limiting the forces to a chamber ensemble and 5 min. pieces. That won't certainly set my imagination on fire.
My 2 cents