When I say Sub-mode, I meant like -Air or -Invisible
Crazy Sub-mode:
This one I think would make people lose their minds.
Make the game spawn double of bosses.
It would be like this: This event would occur around 5 times through the game. It would make more of the same boss spawn.
To make it possible to survive, make sure that no more than 2 bosses go for the same base at same time. (imagine 4 LT.Stealer going for you 
)
I know this sounds too OP and a little crazy, but this could be like a bonus mode! One that wouldn't be necessary to get the achievements.
It would be there just to make the game harder and fun, when people feel like playing with it.
I've read a similar suggestion on some thread "portal is spawn and if we dun destroy it more creeps will spawn and if let on for a
long time a boss will come out?" It could also work that way, The portal would spawn more creeps, and spawn random bosses( or one future boss).
Armageddon Sub-mode:
This one would be an intense spawning of creeps which would occur at any random time of the game. Of course, that would need to be balanced and beta tested.
The idea is, through the game, make some random massive spawning of creeps(stronger creeps), which would make ppl rush to upgrade their defenses.
The number could be (number of creeps)*(number of players) on the game. Also, following the patern of difficult
which the creeps already have
