There's a whole community of folks on Matrix focused on experimenting with realtime document collaboration over Matrix - they hang out at #matrix-collaboration:kb1rd.net. It's pretty early days, but https://github.com/KB1RD/matrix-notepad is fun, and if coupled with some of the stuff that Martin Kleppmann & co are up to (https://arxiv.org/abs/1608.03960 etc) it could be super interesting :)