I figured I'd weigh in a little on this topic, since I'm interested in flags a bit myself.
First, on the matter of the currently implemented ones: I honestly think some of them are a bit ugly and / or impractical. Specifically, United America, the Middle Eastern Alliance, and Asia. The Rev. Countries flag could also stand being changed to something a little more traditional. The New Africa flag, on the other hand, is superb; I don't think it needs any changes.
Second, regarding some of UlteriorMotives's proposals: I like a lot of these, especially the American Union, Russia, and Asian Republic ones. The Rev. Countries one is on the right track, but like Winter said it's a bit cluttered, and the MidEast one doesn't really seem appropriate, either.
I'm kind of tossed up between the two Oceanian flags, though.
As far as the two remaining flags (MidEast and RevCountries), I've worked up a few proposals of my own. Both of them are pretty much reworkings of existing designs that I tweaked a bit to fit more traditional patterns for their respective regions. Let me know what y'all think of them