This sounds rather hard for just fans to create. The website itself probably won't take too long for someone experienced in php, but how are you going to come up with the GBs of hosting space and (possibly) TBs of bandwidth the website will require if it will become big?
EDIT

Maps are 1-100 MBs big, so 20 maps might take 1GB of space. If each map will be downloaded 100 times thats 100GBs of bandwidth

[not in one month of course])