As someone who has been on Discord since the beginning, runs Discord communities and writes Discord bots, here's my list of three things that I believe would make a viable Discord competitor:

- Free without requiring self-hosting;

- Absolutely frictionless community access - here's an invite link, you can start chatting immediately;

- High quality voice calls with screen sharing.

I don't think there is a competitor that hits all three points right now. Screen sharing in particular is often disregarded by developers who have limited interaction with Discord and don't truly understand the platform. It was not an original feature, but it is Discord's killer feature. Because screen sharing is also impromptu videogame streaming.