Good question, I’m just not satisfied with this answer.

Requiring some form of payment is fine, but I’d like the ‘currency’ to be locked into the network so it doesn’t become something you can trade.

Maybe contribute to the network in exchange for a balance, that you can then exchange for domains after which it’s gone.

Ideally also figure out a way to limit people simply creating many accounts and using that, guess you can require upkeep for all domains, so when you stop contributing they disappear.