This is obviously just something I could only dream of but I think would be very cool! If blizzard made the Burning Legion a 3rd faction for players to be on make a few new races but only allow warlocks necromancers and demon's and things that are "evil" to be aloud on the burning legion faction obv. It would deff spice the game up a lot and make for a whole new experience! They could make the new capitol city for the BL in outlands maybe have them take over shatt and that is the new main city for them!
As far as bgs go just keep them the same but have it so its either BL vs horde, BL vs ally or horde vs ally. I don't really think that would be to hard to do as in terms of bases they would just need to re-skin the bases for the BL. I know this isn't really in depth but what do you guys think?