As mentioned before, problem with UCAV is Communication Sattelites which can be downed by ASAT Missiles fired from aircrafts, therefore they would also need to be fully autonomous. I don't think we are there yet, first I think we would see a manned-unmanned plane, possibly in the form of an upgrade to existing stock, I don't exactly see what advantages there could be in this from an aircraft design perspective.
Certainly an advantage could be reduced sortie times, lower costs because of pilots, I don't really see that having any real impact, the cost of pilots I imagine would be very low compared to total running costs. The main advantage I could see would be that every plane could be an ace!