One In The Chamber (based on COD:MW3) is a minigame where you only have 1 arrow but its a one shot kill, and if you run out of arrows you will have to kill them with a sword to gain 1 arrow (you dont lose arrows when you use a sword), each kill gains 1 arrow, and the person with most kills wins, and there is no teams, only you against 19 others (20 in total) and you can say that the one who gets the first 50 kills wins or so, and when you die you will spawn again in a random spot with only one arrow again. ^-^ (sorry if this is confusing XD)
Other large servers seem to have this game although I do admit his is very fun. I think for now, BCSN will try to become more original but maybe edit the game from other servers to make it unique. Originality is what will keep us different from others and provide more reason to join.
I actually don't think that putting custom gamemodes on BuildCraftia will really do anything. I mean, lots of servers use the same gamemodes because they know people enjoy them.