I think that's called Matrix and we already have that.
Anyway, good luck getting Signal to federate with you until you have enough of a user base that they're losing users to your backend. They have zero reason to want to do this, and it introduces some privacy issues (what if your server doesn't respect deletions, etc?).
Anyway, good luck getting Signal to federate with you until you have enough of a user base that they're losing users to your backend. They have zero reason to want to do this, and it introduces some privacy issues (what if your server doesn't respect deletions, etc?).