I don't know if you are planning add fertilizer, but if yes, i have a recomendation:
Nutrient Overload
The Nutrient Overload would happen if you use fertilizer multiple times on a same plant, and it would have 3 stages:
Normal Fertilizer effect
Stronger Effect
Destruction effect
The Normal effect would be the normal fertilizer (Example, Peashooter would shoots lots of peas)
Stronger effect should let the effect stronger and different of the normal (Example, Peashooter would shoot fire peas and normal peas)
Destruction Effect would destroy the plant, causing massive damage on zombies, but giving consequences to the player, from not be able to plant to game over (Example, peashooter would explode in thousands of peas that would get all the map, and your house)
This is just a recommendation, all plants may have a effect on Nutrient Overload, i think this would be a great add to the game
Some ideas of Nutrient Overload
Squash
1. Smash Two different zombies
2. Turns a meteor that smashes zombies on a 3x3 area
3. Smashes all zombies, but your plants gone too, and you can't plant again on the entire lawn
Sunflower
1. gives 3 sun
2. gives 5 sun and the sun production gets bigger (2 sun production)
3. products 10 sun, but explodes and all the sunflowers can't product more sun, this includes new sunflorwers planted after the nutrient overload
