We already have that. Those are the recommendation lists, like the one Primal users are fed during onboarding. Or the follow packs, or shared lists on Listr.lol

The main difference is that a document (list) has no utility to anyone, other than controlling access. A relay is a holistic community of related services, that has limited/predefined criteria for access.

That sounds like a minor difference, but in practice, they're two completely different things.

a list ≠ a server

All relays have lists, no lists have relays.