I once had an idea for a hero. The signature would be called "Let Me in There!" and it would take the Hero and place it on the field. Your health will no longer be able to be hurt by zombies hitting empty lanes. Instead, your health is tied to the plant you placed on the field, because that IS your hero.
"This Plant's health is your heroes health. This plant is immune to tricks that destroy."
It's definitely a cool idea, but what would zombies in empty lanes do? Would they just not hit anything in combat? If so, it seems kinda broken since for 1 sun you can essentially seal off all lanes but one.