Zombie Idea (Would only work if levels are longer

since plants are able to merge I think merged zombies would be a potentially cool idea and some zombies get new abilities improved abilities etc  but to balance it they can only show up after a certain amount of flags depending on how strong they are.

for example 

lumberjack zombie + lumberjack zombie can create a super lumberjack zombie which can deal damage in a 1x5 area but can only spawn after the second flag or volcanologist  + bowling zombie make bowling volcanologist throws bowling balls at the second furthest plant after second flag magma zombie + magma zombie = volcano zombie when damages releases lava in a 3x3 area damaging plants over time can spawn after wave 3

there can also be vaiants for the basic zombie but is not as effective

examples

lumberjack conehead uses a axe to attack plants and deal damage on 1 tile (after wave 1)

stealth buckethead: becomes invisible after 4 seconds when not attacking (after wave 2)

also if two different zombie are combined you could have both versions

example

lumberjack + bowling zombie 

Lumberjack bowler throws axes at plants dealing damage in a 3x1 area (after wave 3)

spiked bowler zombie throws spiked balls that deal damage in a 3x1 area (after wave 2)