This tower defense idea was submitted by Adrian.

Towers that have an area effect blow back. (low fire rate, no damage, pushes ground units OR air units back, directly away from tower.) One can imagine having to crack one’s brains trying not to have these towers push creeps forward when pointed in the wrong direction.

Plague damage. Affected units cause damage to units around them in aura radius and to themsleves. This damage is proportional to plague strength in each unit. Plague strength in each unit drops over time, is refilled by being hit by another plague round.

Insanity attack. Hit creeps will have a new timer. This timer has a random time from 1 to 5 seconds DURING WHICH CREEP MOVES FASTER; when timer expires the creep starts jiggling on the spot, doing an amount of damage over time proportional to its hit points to a very small area of effect around it.
This insanity period is also random, from 1 to 5 seconds.
Potentially damaging when mixed with other slowers, potentially really bad if towers start shooting at the wrong place.

Teleport tower. Causes creep hit by “cause teleport” tower to jump to a corresponding “teleport destination” tower. Creep also suffers 2 seconds of disorientation.

OVERALL MANA BATTERY: All towers use up mana when firing. More powerful effects use up more mana. All towers can be set to “on” or “off”. Mana battery upgrades cost gold. Upgrades increase battery size and recharge rate.

RANDOM IMMUNITIES PER WAVE. Listed above are 6 to seven effects; mixing immunities to different things will cause people to crack their brains and at the same time become hopelessly addicted.