There are many Discord 'alternatives' but what if instead of trying to replace it with something that (kind of) works the same way, we actually got down to JTBD and first principles? To me, Discord is a hot mess, UX-wise. I've been working on this problem for years and it's so interesting what we think we "need" versus the anti-patterns many of the features come with. Considering holding some UX-focused hackathons to imagine better approaches, look at folk technology dot org.
A lot of the features of these applications, be it Discord, or Slack, or Teams, exist because they want to lock their clients to their platforms.
Ideally, a Discord alternative server would be self-hostable and focused on providing messaging. A client could connect to multiple servers at the same time.
I still don't get Discord. It's difficult to understand and use, and this from a tech guy. I kind of miss Teamspeak. You could chat or use voice. It worked great... Discord seems way too complicated to me.
It's all variations on IRC. I'd think something resembling Google Hangouts for the audio/video aspects of Discord, with threaded IRC with history for the text chats would almost completely cover all use cases. A Discord-like client would join the two services under a single interface and identify the user with a service-appropriate identity.