i'm hoping to soon be able to release something that does exactly what i'm describing, as well as integrate better with lightning

so users will be able to DM the relay bot, get an invoice to pay for a subscription, pay it, and then add it to their relay list

the relay will have a background process that keeps a second database that tracks users query activity and registers it in a decaying list that refreshes the expiry each time the query asks for a given npub, it will combine that with the user's follow and mute lists, and it will pull all data relating to conversations with followed and not-muted users, and the threads connected to them

probably will also want to create a special "archive" configuration as well, that you open accounts with relays running these nodes that just pulls everything in a firehose and stores it on a gigantic database, maybe using a two level cache again (the fetching process will require a level 2 data interface that does the forwarding of queries when data is not found in the local store, and caching it as well as returning it from queries)

these archive relays will be able to charge a higher fee to relays implementing this, and it will effectively mean that one need only pay for a couple of different paid relays and the network of connected relays will become your secondary data store and everything relevant to paid users will be kept hot and updated immediately