The point of a CMS like this, is that it would be built specifically for people that write client side UI's. So there really is not a need for a server run-time component, because the client app, request data and assembles the view. But there is still a portion of these web apps that are static, like the header, footer, etc. Say I want the same header on index.html and page2.html, I have to accomplish in one of two ways, I either use something like tiles for Java or I used something like dust.js on the client (for this style app, tiles is out, so it would be dust). What I would like to see is a CMS that composites that header into both pages and removes it from a run-time concern on either side. Now that's just one really simple example and I know current CMS's can do that, but I think their is a value in a CMS that only caters to this style app development. Further, the reason, that I would like to see it not have a server component is by eliminating it, and putting all of the dynamic CMS logic in a JavaScript files that get's delivered to the client, would be because CDN's could then be exploited for scalability. I could see, a pattern where companies use a CDN for all of the content, and an AWS instance to set up REST services and have virtually no infrastructure.

What about static templating like mustache?

Sure and that is what a lot of people use, but I would like to see something that allows copy editors and designers the ability to publish without the aid of a developer, after the core application has been built. I would like to see the traditional CMS workflow adapted to modern client side browser apps. But I think there is value, in a clean code base that is developed for this market and not a CMS that has been adapted. It's a completely different way of developing apps and I think we need a new CMS that is designed specifically for it.