Ha thanks, we’re slowly edging closer. One of the steps was to make existing vector files a Tables.jl table, which is now done in Shapefile.jl and GeoJSONTables.jl. From those I hid the geometry column to the Tables interface because I want to treat it specially later. We will need some work on an improved GeoInterface that @yeesian has done some thinking on lately. Then based on that interface perhaps we can set up a generic GeoTables interface for this kind of functionality.
Although besides the generic interfaces and types it might be good to also start working on a decent GeoDataFrame that just takes for instance a DataFrame and a vector of GEOS(?) geometries, and start defining some operations on them.