I really feel like a straightforward self-hostable (ideally something like a single static Go server binary), simple small group (10-1000ish users) Discord-ish (text) chat system should only be a few weeks to MVP at the outside.
You know, accounts, channels, messages, basic ACLs, some limited image attachments, and the like. Sort of a midway point between IRC and Discord. Enough to get going.
Mobile clients (which I expect many would want) would be the biggest pain point.