I think a tradition is official on the 3rd time.
Do something three years running and you have yourself a tradition.

As far as the contest goes, you could ask for traditions on certain topics like food, games, music, most bizarre.

Another idea would be a what if scenario. Like what traditions could you do with relatives from 1800's if they were alive today.

That's all I have for now. I'll let you know if I think of any other ideas. :)