I would like animated tokens and props. You already support PNG so APNG would be a good choice, with its built-in transparency functionality. When you click on it you would have 4 options instead of just Delete or Toggle Visibility. You would have Animate Once and Loop. I would use Animate Once to make a door open. I would use loop to loop flames on a firepit, bubbling lava, or windblown leaves. These would be amazing additions for VTT use.