I feel like the average person isn't looking for something professional grade, sadly it's hard to get people to go away from Discord at the moment. Hard to suggest alternatives if people aren't seeking them yet.

If I had to say it would have to be something customizable, letting a user to delete their data even after getting kicked from a server, very fast and seamless joining process ,great gif/sticker support without any premium features etc. But really that's just some fantasy app lol. Discord is doing just fine destroying itself however

By the way, I didn't know there was an instant online test app because when I searched for Zulip I was in the download page and it doesn't say anything about trying it online. Seems like a strange suggestion UX wise but that's how I feel about it (wonder how many people missed out on this?), same thing after you enter the app. It should have a test area for the new user to chat around by himself with a bot or something with locally/session stored messages.