Idk if this possible, but if it is, then it's a very good idea.
Make bdw not between human and orc, but between everybody. That would fix the unbalance between ppl who register in diff nations and only 0-1 player would stay out. For that to be possible you need to make a program that would recognize your party as friendly and others as enemy. Hope it is possible

Also you should program the algorithm not to take more than 3 players of the same class. That would remove the 5-6 priests in one party problem.