With the release of the Roll20 apps designed for playing games face to face, I feel like more than ever a Roll20 open API would be awesome. I run a game at my house with a pretty fun setup that has Roll20 at it's core. It's been seen/done before, but I have a projector the displays a player view onto the table, and a laptop where I control everything from. I also have a raised platform on my table that has controllable LED strips underneath. Each LED is individually addressable so I can do things like torch effects and other lighting situations per player etc. and I have an app I'm developing to go with it. It currently controls the table via wi-fi using a mobile app. If Roll20 had an open API I could have what's happening in Roll20 (and on my table) affect those lights via my app. Some simple ideas are things like flash a players lights when it's their turn, activate torch lighting when I give them a light radius, turn their lights red when they're bloodied, or a million other things. There's a ton of possibility.
