Arakkoa would be really cool, birdlike people would be something completely new since all the current races are mammalian. Of course their playable model would have to be completely redone, just like the goblins, worgen, and draenei were (blood elves too, although theirs was just a peachy colored night elf and was technically a high elf,) so saying that they are too ugly doesn't make much sense because other previously unplayable race's models were ugly and they all look fine as playable characters.
You could customize their feather patterns like hairstyles, some would have short little feathers that would look better on melee classes, while there would also be longer feathers like the ones on the current models. They could have scraggly, styled, smooth, etc feathers, customizable beaks, and there would be lots of color options. The playable models might be more upright and not have the crazy fingers (at least not while standing normally, maybe as an idle animation) and would probably be around the size of a male orc or just a bit bigger.
The Raven Lord doesn't make sense as a racial mount, since it's a creature who's worshiped as a god. I think a basilisk or warp stalker would make a better racial mount since they are normal animals that are running around everywhere in Terokkar Forest where most of the arakkoas are.
Vrykul though, I'd rather not see as a playable race. They are really cool and have more lore behind them than arakkoa, and their architecture is nice looking, but there are already a lot of playable races similar to humans and having another one, even if they are on the Horde, would just seem boring when there are plenty of nonhuman-looking races to choose from (or create)