how about a hardcore mode, when you completed a level you can play it again, but this time the emitters spam 2x the creeps :) or 3x ;D
this is already in the game it is called double down and you unlock it after beating the campaign.
not after beating the campaign, after beating the campaign, the conquest and the special ops
I can be wrong, but I think finish special ops is not necesary (I have double down long time, but I finished special ops at last week).
i mean when you completed level 1 you can play it again in hard mode before you move to next level and so on not when you completed the whole game
well playing hard mode is kind of a reward for finishing the maps, and alwego i believe you have to earn all the achievements in the hall of honours which involves beating all the maps, where's virgil to clarify when you need him? lol
To unlock double down, you have to basically play every mission (to get every achievement). Once you have done this, the DD button will appear.