I too believe having an open source alternative to discord would be greatly beneficial (I for one don't have any will to join discord). What about matrix https://matrix.org/? it is a mature protocol with good clients, including web.
Regarding the AGPL, I understand there is a continuous debate between the permissive and copyleft licenses camps, but I don't see how that would impact we using something under the AGPL, as long as you don't modify the code or distribute it you don't need to AGPL anything, and isn't 0ad engine GPL (great choice imo)?