I think support for multidimensional ratings would be a game changer.
Instead of forcing everyone into a binary or handful of reactions to a post, imagine if you could rank something as funny, what language it's written in, the quality of the post, if it's spam, if it contains a picture of kittens, or any other thing the user cares about.
I forgot to call out that Matrix(protocol) seems like a worthwhile call out in looking at useful core technologies.
A backend only API so people can use custom clients
I am not enough of a Discord ninja to really provide an apples-to-apples of many of those things, but I can say with the highest confidence that Zulip addresses #1 in such a stellar way it actually pains me how much the open source community ignores using it in favor of fucking Slack or fucking Discord, where, yes, information goes to die
I also believe they address the P2P video <https://zulip.com/features/#:~:text=Voice%20and%20video> but they offload that to Jitsi rather than trying to reinvent the wheel
They don't even have to host Zulip themselves, as zulip.com offers free hosting for open source communities. Just pains me