That’s a feature I’ve dreamed of back when I was working on Hydrogen (GPL’d multiplatform drum sequencer), since I used to work on tracks with friends, but they moved to different cities - no idea how hard it would be to implement, though:
Basically, I’m talking about “online multiplayer” - two or more people connecting over the net to work on the same project at the same time. Everything one user changes will be instantly visible/ audible to every other user, samples one user loads will be copied to other connected users etc. VST/ AU plugins obviously won’t, but , you can talk about what both/ all users need/ have before loading a plugin so that shouldn’t be an issue. If users decide to continue working on a project on their own, the next time they connect, all other users will be able to preview the changes and decide to sync the project or revert to/ use their version. I don’t think there’s a need for advanced communication features, simple text chat should be sufficient - for voice chat, there are more than enough external cross platform applications like Skype or Teamspeak.
Another feature I’d love to see would be versioning (incremental saves with some additional metadata, stored in a “virtual directory”), but that’s probably something for a different post.