Should this be a new BIP? I know netki's BIP75 is out there - but I think it's too specific and too reliant on the domain name system.
This is not quite accurate. BIP75 is designed to be independent of any name resolution system. You could use it with a static URL that you share, for example, or even use it to implement a mesh-network payment system over bluetooth. Netki's wallet names do use DNS, but that isn't related to this discussion.
What BIP75 *does* do is provide a way for a client to get a new payment address for every payment. I personally think it is better than BIP47 for the uses you mentioned (subscriptions, etc).
I'm glad you brought up identity methods other than x509. At breadwallet we are thinking about how to establish the most universal system, and letting users identify themselves with any of a selection of identity systems is ideal. I think the pki_data slot should be constantly expanded to allow new identity types, but they should be explained/standardized in the BIPs that add them and use universal names. "netki://" wouldn't be appropriate, for example, if their method is open sourced and possibly used by others--it should instead be given a product name like "dnswallet://" or something more clever.
James