Make a web browser using webkit and Qt. Host it on freedesktop.org and incorporate anything and everything to make the browser integrate with freedesktop. Using Qt the browser would look good in Gnome and KDE with the Plastique and Cleannlooks themes. DBus support is a must, freedesktop.org mimetype support, and more. Create a standard for storing bookmarks. Use xdg-user-dirs to place things in the Download directory. Would it work? Would it take off? Probably not, people like to bicker to much. KDE users wouldn't use it because it isn't Konqy, Gnome users would refuse to use it because it has Qt. Would it be fun to make? Yes.
Edit: So I ended up kinda doing this one: ttp://arora.googlecode.com/ Rather then for freedesktop it is more cross platform.