Easily Tengu. It'd make perfect sense for there to be a lot of them, since as far as we know they have an entire booming civilization inside their walls. All that'd need to change would be to have something happen to convince them to open up to the outside world and venture out themselves. They have a unique culture that would be something new and interesting for players to explore, but not TOO unique to the point that it'd be out-of-character for them to be player-driven characters. Having someone like, say, a Kodan running around doing some of the stuff player characters do would be strange considering their religion / philosophies. Their fighting and lifestyles also mesh up with player characters. They fight using all variety of weapons and we've seen them as craftsman and traders as well. Their civilization is right up against some of the current maps, which integrates them nicely into the game world rather than shoving them in some far-off corner of the existing map disconnected from everything else. Fantastic customization options and overall variety. Outside of sharing a similar hunched-posture to Charr (well, some of them anyway) they have a very unique anatomy from the other playable races. And just being based on birds means there would be a TON of inspiration to draw from for faces / colors / patterns, as birds as a whole are some of the most varied creatures on the planet. They could have color palettes to rival Sylvari, and could even have their own signature style for hair / feather styles, derived from Asian influences. Plus they were supposedly planned as a 6th race for the original launch at some point, and even if it didn't go very far into development they've probably at least got some resources they could pull from to aid in creating them now. Even if its just concept art.