@lumi i'm not entirely sure how signal's architecture would support full decentralisation, and the phone number requirement is likely to prevent spam accounts from being generated by the millions. they've made some very careful tradeoffs to get what they have and i think they're very fair ones.
what you may be asking for here is a kind of E2EE IRC, which afaik does not exist yet (and ergo would have to start from scratch), and would put a heavy burden on relays that need to forward n+1 anonymous packets around indiscriminately.