Holiday themes are a good one. One of my favorites during my time on YCM was the creative secret santa, where someone who try something they haven't done before in a sort of gift homage to someone else in the pot. It was a nice gesture and didn't put too much weight on the more established "creatives" in certain mediums. Say NightWalker, famous YCM GFX Daddy, made a dedicated OC of a character from an anime even thought he wasn't a weeb. Things like that were fun.
Community activities are ones that help to bring people together. Tournaments do not have any lasting effect on this site, except for CardMaking, and those were structured well but had a sense of elitism rather than a community following. That was more in how they were promoted instead of trying to increase the creative outflow or the proper design philosophy of cards.
GFX is dead. Don't even bother. If someone opens up photoshop, send in the FBI. Stick to instagram editing, that's where the real money is.