Signal doesn't suffer anything worse than DoS if a hostile party controls the central service. That's its point and role. It's based on the assumption that such hostile parties as governments don't like DoS'ing central services, they prefer to be invisible.

For other points and roles other solutions exist. One can't make an application covering them all, that never happens.

Briar again (I've finally read on it and installed it, and I love how it works and also the authors' plans on the future possibilities based on the same protocols, but not for IM, say, there's an article discussing possibility of RPC over those, which, for example, can give us something like the Web ; I mean, those plans are ambitious and if I want them to succeed so much, I should look for ways to defeat my executive dysfunction and distractions and learn Java). Except it would be cool if it allowed to toss data over untrusted parties, say, now if two Briar users in the same group are not in each other's range, but there's a third Briar user not in that group between them, their group won't synchronize (provided they don't have Internet connectivity). If one could allow allocating some space for such piggybacked data, or create some mesh routing functionality, then it would become a bit cooler.