Hello again Sjors! To introduce a new tag or some other protocol improvement, you can make a PR on the nostr-protocol/nips repository. Then, to get it merged it typically needs a general approval and to be implemented in at least 2 clients and 1 relay.
Having been in many geo discussion, I can tell you that privacy would be the top concern for having a wgs84 tag that pinpoints a location. Most photo uploading services on nostr (if not all) scrub exif data for privacy. But I can see legitimate reasons for broadcasting a coordinate, so if you can assuage the privacy concerns then it could be a useful addition.
I will mention that in the deep github lore around the g tag I recall it being said that any geo data could be put in there, not just geohashes. Nobody does this AFAIK, but you could technically just put a wgs84 coord in the g tag.