the only thing that tox is missing conceptually in the current protocol is a storage facility, so that true offline messages can be implemented (right now both parties have to be online at the same time eventually for "offline" messages to be delivered). but that's the only missing thing that would be expected from a proper messenger, really.

and as soon we're talking storage layer we're in storj/maidsafe land. maidsafe claims to work without pow/blockchain. their security is based on a "proof-of-resource" and a node ranking system (there's still debate going how/if it can be secure enough). you can run a farmer though on your 24/7 online desktop box to provide storage space for others, to earn some of the integrated safecoins. but your devices will run under your same account, so apps could provide very much convenience in such a system, much more so than in today's internet/web where you need to maintain hundreds of logins/passwords.