OwnCloud's plugin architecture allows for all of these things mentioned. But plugin authoring for OwnCloud is opaque and tedious. There's a Hello World and a few examples, but the environment isn't fun to work around.

This all could be accomplished with their api and your own web-server. The coding would be the hardest part though from an effort standpoint.

I actually think DB should be the webserver in this case.

Why can't I just make a "web-server" folder in my dropbox, drop a bunch of html, js and css and a few image files in there, get a URL and voila, instant website?

You sorta can already in truth, but they don't like it and the bandwidth is pretty severely capped on these kinds of sites. Make it part of the paid account (with x-fer up to 1GB/mo or whatever) and have high bandwidth users pay more?

Those ideas tempted me to stop what I was working on and try one of those ideas though, it was a great set of ideas.