Elements that know they're user-editable (images with upload controls attached to them to replace the image that's there, dates that trigger calendar controls when clicked, and inline editing for other elements that actually works and is consistent across browsers).
An offline database in the browser that has the concept of user accounts and resetting passwords baked in, as well as subscription payments, and has a universal API that can be automatically deployed to any hosting platform.
All of this would make building web apps and games trivial for so many more people -- write some basic HTML, upload it to one of a million platforms of your choice with a click, and you have a testing ground for a product that can grow.
It would be a way for anyone on the internet to build something useful and get it out there without learning a thousand technologies with a thousand nuances.