Okay, this is going to sound a little odd, but I think support for animated tokens - perhaps most easily in animated GIF form - would be a huge asset. I tried importing some animated GIFs of sprites from games, kind of bouncing in place in their fighting stances for something of a "Final Fantasy Tactics" feel, which would make it really easy to pick out active tokens, but alas, they did not animate.
I don't know how Roll20 implements images, so I don't know if directly supporting animated GIFs is feasible, but I think support for animated tokens would be a good idea. Not only could they be used for player characters, but for environmental features, like filckering torch flames or subtly glowing cave mushrooms. If you could make some system of sequencing alpha channel PNGs together in an animation, I imagine things like glowing auras and animated spell effects would add a lot of zazz.
I think even if they were tricky to implement and even a bit proprietary, animated tokens would fetch a hefty price on the theoretical asset market. I think there are lots of DMs who would like to place a shimmering, crackling arcane summoning circle in the middle of their climactic encounter to stop the evil archmage from summoning horrors from beyond the reckoning of mortals.