Ask HN: An Open Source Web Game Engine?

So one project I'd like to work on over this summer (Australia) is putting together or working on a web game engine (Javascript / HTML5 / CSS) that is similar in design to Wordpress. In that it has hooks and filters that anyone can use to build plugins for the engine. So for example if you were developing a game with this engine and wanted Facebook connect support you could simply grab a plugin and use that and it would work with your game, or if you wanted Paypal to process microtransactions you could get a plugin to do that etc. The problem is I'm still not sure how this system would work. I'm thinking it'd have to have some sort of PHP backend / control panel similar to Wordpress where users could add the plugins and change options related to the engine. Then building a game would be similar to building a theme (but of course a lot more complicated) and it would run on top of this framework. I know this is a big complicated idea but I'm sure there's quite a few smart minds on here who might want to flesh out and run with this idea and develop a truly awesome open source web game engine to make it super easy to build web games. Thus my question is. Does anyone know of anything like this? Or do you know of any good web game engines currently out there that could be improved upon to make them more modular so anyone can develop plugins for them etc?