How about 3-5 weeks dedicated to a specific fighting game character or a game?
Nothing like a real contest or a challenge, more for the sake to draw something fighting game related and see the results of others doing the same one as well in just a week?
I just did a "draw a goomba" week in my other group game-art-hq and the result were that over 20 artists, ranging from someone who is new on dA to someone who is here since 9 years and has some million pageviews participated and had both fun and created nice takes on the mushroom enemies of the mario games.
Something like that, dedicated to a fighting game like clayfighters, or a specific c