We would have to sit down and actually think what an opensource solution can achieve and how it gets traction. Also from the get go it should be clear that there will be no feature parity between it and discord. If it was me, I would cut out the whole chatroom functionality, leave private messaging in, use threaded conversations as a standard and but a decent videocall system on top. But this would be my version of it, other people have other needs.

For the video call system you would not have to reinvent the wheel, use something existing like Jitsi (?) or alternatives. Then you would

Maybe the best bet is to look at matrix and wrench out the chatroom focus and replace it with threads?