The new Bat Swarm mount is very interesting and unique. I love it. Other mounts along similar lines would be awesome too. Such as Autumn Leaves or Snow Swirls or even a Blossom Petal mount could be really pretty. The idea that we are powerful enough wizards to turn into a bat swarm or just blow along as a cloud of leaves or snow or blossom petals is really cool. :)
I would personally love a Butterfly Swarm, imagine you transform into a swarm of colorful and graceful butterflies (well the animation is the same with the bat swarm).