Hey i was just browsing and thought... well CoD6 had the tactical nuke..
1. Could put a buy-able nuke into the game but with a big price obviously maybe like 150,000 or even 200,00. It's properties should be able to clear all enemies off of the map and maybe your own team mates as well, so might have to make a buyable bomb shelter also xD.
2. My second idea was to have melee weapons, such as the classical Ice Climbers' hammers. Sorry I don't know if your characters are based off of the Ice Climbers but if they are then this could be like some sort of special to unlock for your character. Even if they aren't you could still implement the hammers or other things just like from the original Seal Hunter's highscore list where there is the guy clubbing the seal with different melee weapons
These melee weapons should be able to do massive damage, or then do other things such as flipping the enemies to move 180 in the opposite direction ( just like how Mario's cape thingy in Smash Brothers does).