The project is open source and open to contributions from anyone. Every single time this comes up the project contributors have said that they will gladly modify <amp-ad> to support any network they can. They're also constantly developing new <amp-X> components to fill in the gaps for other embeddable services. You want <amp-vk>? Submit a PR! Open an issue! It's open source!
And please, please tell me what's wrong with using e.g. <amp-youtube>? It's just a web component! All it does is what you want it to do, and if it doesn't do what you want then either you shouldn't do that, or you can just submit a PR.
The whole point of AMP is 1) make sure the developer doesn't do anything that'll make the page load unnecessarily slowly or repaint/relayout unnecessarily and 2) make it way easier for the developer to accomplish #1 because all you have to do is type <amp-youtube>.
AMP is no different from, say, jQuery, or React, or Angular. It's a different way of doing things, but it's not a "gatekeeper" on the web. It's just another option, one more tool in your belt, if you so choose.