This would be great if it could happen, but there are compatibility issues...Currently, dag, dries, freshrpms, newrpms, and atrpms are working to be compatible but may have conflicts with fedora.us and livna (in package names, dependencies, etc.).
Please correct me if I am wrong here, but this is what I have gathered. There is sort of an idealistic rift in that fedora.us strives to be a community project but because of that it is slower to get new packages out and it only carries "mainstream" packages. Dag, freshrpms, etc. are individually (or almost) ran and have many more obscure packages and ones that Fedora can't officially include like xmms-mp3. But because they are individually ran, the rpms are not as well tested as those from fedora.us.
So I believe this is a good idea, but to an extent it is already in progress with projects like DAG"s repository merge at http://dag.wieers.com/home-made/apt/mega-merge.php
and Fedora Tracker at http://www.fedoratracker.org/.
And then you have the small, specialized repositories like the flash-plugin repo that may jump on the idea of a contribution-based repository or they may prefer to go it alone.
So after my rambling, I think that the central repository would be very difficult to pull off. There seems to be too much disagreement about dependencies and organizational structure. For example, I prefer to use DAG, etc.. but I don't care for the atrpms package that atrpms makes you install with any of their packages. And there are so many kernel packages out there like ccrma that wouldn't really work with in the main repository. It would be a task to find a way to get everyone to come to agreement about this, especially the already established repositories, and make sure things from outside contributors remain compatible with the rest of the packages.