Firefox, Chrome, and Opera can use the new WebRTC protocol to do peer-to-peer voice and video chat, which might be really cool for multiplayer (or might prove not as versatile as text chat, which we have right now). Given how many frameworks there are that make it easy to try out, it's probably worth an experiment! Here's a semi-recent list I found. @sderickson also saw interesting presentations by XIRSYS and OpenClove a few months ago.
This is, of course, an enhancement for CodeCombat's multiplayer. I'm thinking that even if it doesn't make sense to use the video chat, it could be fun to add voice chat so that players can play together more like a game. We'll just have to try it out and see if it's fun.