I think if we do a flexible system with free form we should also define in the nip the conventions that a bunch of nostr apps are using in order to make it easier for those who want to support new fields to know the naming. 

Like for the ethereum folks is it eth.address or ethaddress or eth_address? nostr:npub1q3sle0kvfsehgsuexttt3ugjd8xdklxfwwkh559wxckmzddywnws6cd26p ’s proposed nip should be popular because it’ll allow support for linking to every kind of crypto wallet address. ;-D