How about modification of the month? It has loads of advantages:

1) Everyone can get involved, if they do something new they can enter.
2) It encourages people to push their car forwards and encourages unique and new thinking.
3) It gives people an extra reason to post up photos of their latest change. I do loads to my car and don't bother posting pics up and i'm sure other people are the same.
4) The same cars won't win over and over again.