You can build up a project, if you specifically adress
the advanced and practised users here. (Skripting, Modelling, Textures, Leveldesign)
but they will not really learn much more by doing this, so there is not much motivation.

If you asseble some newbees they will lean much more, but probably not getting
anything that will impress the crowd, so they will loose motivation also.

If you mix advanced users and newbees, there will only be discussions about the quality,
and the advanced users will throw out most of the newbee stuff in the end.
This project would die pretty soon also.

Projects need to be conducted by a small team if you really want to make a GAME!
If you just want a collection of textures, levels or models, a contribution pool would work.