I suspect it would be easier to create a super app that extends something like Chromium or Firefox with additional APIs to access all the "fun stuff" a phone has to offer beyond what's already available via standard Web API's. Considering FireFox WebOS's approach to the design of these additional APIs would be worthwhile. Then WEMOS apps (web only mobile OS) could run under this super app. The super app would need a system to keep track of which WEMOS apps have permission to access specific API's on a users phone. Would also need some kind of WEMOS app discovery system. A way to initiate running WEMOS apps from a regular web browser would be convenient. Initially users can run both native apps and WEMOS apps. Once user and developer buy-in is high enough a custom Android ROM that preloads only the super app with WEMOS apps to control the device (i.e. make calls, send texts) could be created.

I don't know whether Chromium, CEF (Chromium embedded framework), Firefox, or Firefox OS would be a better choice. It worries me a bit that Mozilla doesn't have something as mature as CEF, but that's just me speculating.