well, if a relay is to spec, it should not store anything with kind 20000-29999 and these can be used for not just coordinating messages to the relay but also synchronous comms between users
i could talk about how it would be useful to integrate some client elements to relays so they can be seamlessly deployed as a personal system-wide local cache and then you get your "your relay your data" state management as well... this has been a key target for my work with #realy and why its access control system repurposes follow and mute lists