What should my future browser do?

I was wondering if Web browser capable of installing javascript extensions like Ajax framework on top of existing runtime (it is little bit more than js caching) with its own security and other aspects built around, we can avoid multiple downloads many a time and go on building rich internet applications without worrying too much about size of the scripts. Browser should be able to

1. understand different versions of javascript applications
   
2. use requested version and able to download like plugins if it is missing
   
3. Multiple threads for processing html/js/css or dom updates
4. use these extension across different domains, so javascript extension and html page should follow set off rules to share across.
   

this kind of feature will enable creative people to develop rich features and perfectly make the browser as a platform.