While I think it would be fun, keep in mind that none of the classes are really balanced for dynamic group compositions in PvP, which is why the current compositions are super restricted, and some of the classes have had their abilities tweaked specifically for PvP. Can you image three SMN, three BLM or three WAR in a 3v3 match? I know this sort of matching can (and does) work for other MMOs (WoW comes to mind), but that's because the classes are balanced for 1v1 play (to the best of their ability). Also, in arena in WoW, your only job is to wipe the other team... there are no real medal objectives.

While I would LOVE some variety in ranked battles, it would require a LOT of work from SE to make it possible... waaaaay more tweaking than was already done.