The closest realistic option is probably XMPP and Mumble. I just think both need a modern UI that supports everything that Discord offers OOTB. I am working on a client for XMPP on the side, nothing special yet, but I can login and chat with myself on a prosody instance, so there is that. Would love to work on open source full time, but it never seems to pay the bills. I do intend on eventually open sourcing my client.