This one single feature could potentially have the largest impact on my workflow in 2018. As much as I love async task management. Real-time collaboration with remote team members should be fascinating ;)
I guess the next step would be something along the lines of Google Wave where you could playback the "conversations" and see the learning process develop.
Even as the author, I think web-based IDEs have pretty limited utility, but it is a great showcase for the power of our API. We (my co-founder and I) built this in about 10 days.