That could be possible. Will need to balance between customisation and user experience. Making it as easy as possible, and also performance.
Discussion
Absolutely, and I would assume it may take a few passes at it before getting the right balance and feature set.
Not sure where I would prioritize it either. NIP-07 and NIP-46 for all clients are more important to me right now. Relay controls of some kind would be close behind it, though, for those clients that really don't support it. I admit I do also like and frequently use the Primal model, which scraps relays altogether and basically just runs its client on top of its own relay, as I understand it. Maybe there it would be even more important to implement lists, as well as external mute/block lists.