I think this is fabulous ingenuity, useful fun, and the eccentricity doesn't rock any boats along the way.
We're currently considering adding some sort of backscroll support to IRC (maybe automatically setting up proxies for users), or installing Rocket Chat or HipChat on a local server.
While there's not yet C64 support, Mattermost connects to IRC, Slack, Gitter, Discord, Telegram and HipChat using Matterbridge (https://github.com/42wim/matterbridge).
In terms of features, the Mattermost community has added quite a few that aren't available in Slack--markdown support, multi-team accounts, threaded messaging, etc.: https://www.mattermost.org/what-slack-might-learn-from-its-o...
Mattermost deploys as a single Linux binary with MySQL or Postgres and you control everything.
It's boggling.
Slack is a glorified instant message service, and instant message clients have been handling WYSIWYG for a couple of decades now.
This kind of thing can also be annoying in sites like Hacker News--e.g. when I try to use an actual asterisk a couple of times and the damn thing italicizes a big random chunk of my post instead--but HN at least has the excuse of a consciously minimal interface. Slack coats itself in slick, modern UI but makes you type emphasis like it was Usenet. It shoves giant obtrusive thumbnails into the chat every time someone posts a URL, but doesn't let you make an actual hyperlink. It's minimal where it should be complex, and complex where it should be minimal.