I’m a developer for a fitness tracking app and we’d like for our Twitter cards to feature a thumbnail of the user’s GPS track overlaid on a map. It would be a more engaging experience for the user, as opposed to featuring a generic icon in the summary card.
It is against the terms of service of pretty much every mapping provider out there to proxy content rendered from their service. Even if we negotiated a deal with such a provider and managed to serve such thumbnails from our servers, Twitter itself proxies the image and we would be SOL.
In their Opengraph support, Facebook did add some limited mapping features that respond well to such use cases — see https://developers.facebook.com/docs/showcase/runkeeper/ . I’m hoping Twitter could follow suit and provide at least an equivalent to this, if not a better designed solution.