@thisisaaronland @Edent for geotagging specifically, it would be nice to use Curies for the values so you could use different place vocabularies.
So, here, I'm showing my location using a Wikidata item.
ap:location=wd:Q3304955
And here I'm using an OpenStreetMap relation:
ap:location=osm:7950480
However, I'm not sure the value parts of machine tags are allowed to be that rich.