r its used for references (urls, etc) and g for geohash (location), its true that nip10 originally just try to create conventions around the replies, but since the r and g tags add the context for a note can be interesting to consider add a convention for kind1 that contains it. The only client that do something around that its amethyst for example that shows g tags if an event contains it 

 Interesting, does not seem as urgent as "a" but maybe add this to the PR? https://github.com/nostr-protocol/nips/pull/809 

 Yes, definitely a tags are more accurate for the actual approach of the nip and have loads of sense. But yea, it might be interesting to add these others too.