It’s not going to happen. If it is open enough that anyone can upload work, then it will quickly be overrun by average and below average photographers. This will make it useless for a busy person trying to find a the right photographer without wading through all the noise.
If it is not open, well then you have the same problem you currently have—getting the attention of the gatekeepers. There are sites that get regular visits from art buyers and editors like http://www.featureshoot.com/ but getting featured by them is not much easier than getting picked up by a magazine.
You need to find your own way to stand out and attract the attention of those who assign the kind of work you want. Any website where you can walk in and upload work that promises this is lying. Hoping to get assignments by uploading to websites is probably not a good strategy.