Interesting idea. Two thoughts that come to mind after reading your post:
(1) Parameters for styling. Besides being able to use CSS for defining the height, width, positioning, etc. of the ‘geo’ element, as a web designer/developer you might want to have some influence on how the map itself looks, in case you want to match it with the design of your site. For example, whether to use satellite imagery instead of a map; if names should be displayed for streets, buildings, areas, etc.; colouring for roads, routes and other elements.
(2) If the ‘location’ attribute is to accept any data, maybe a (non-intrusive) fallback needs to be defined in case the mapping app fails to match the data to actual coordinates. It would be a bad user experience if you open a website and you get a pop-up from your mapping app saying “location not found” because of an embedded map.