It is obviuosly London at the end of 19th century, right? I had this idea for a while, actually. But let's start from the begining. Each world have: 5 unique type of zombies, "dash attack (exept Wild West)", something unique on the lawn, special mechanic that may or not may relate to lawn.
So...
Dash attack and "something unique on the lawn" - each level have several manholes, you can't plant on them, but different kinds zombies could show up from it. Manholes could appear pretty much anywhere, even on second tile from left.
Special mechanics: Just a fog is kinda boring for me, but what about fog AND rain? So first part is day and contain rain mechanics, second part - night and fog mechanics.
Rain happen several times during the level. It makes sun not fall from the sky, make fire plants weaker and the most important - change zombie behavior. During sunshine they are attack based, during rain - defence based.
Besides normal zombies and Gargantua-policeman we have:
Zombie with umbrella. He is tougher and faster than usual. When he reach first plant umbrella zombie smash it and then walks normally without it. Umbrella stays on the ground, just like surf. However, if he still have umbrella during the rain, he opens it and walk normally. Opened umbrella have big amount of health and protect all zombies on tile from catapults.
Zombie detective. He is also tougher and faster than normal zombies. Detective hold magnifier in his hand and use it to burn plants, jut like archeologist. However, he have different weakness. first: Magnetshoroom. Second: magnifying grass (he just reflect sun back to zombie). During the rain he starts crawling instead. It makes him slower but immune to most shooting plants except catapults.
Tea zombie. It's a passive zombie. He holds teapot in his hand. When he loses hand, teapod explodes and tea spills to near zombies. It makes them move and eat faster (remember potions in Dark Ages?). Weakness: Complete freeze. Cold plants also help but you need some time to cool him.
Second part contains mist and night. Zombie sleuth can only crawl now. Fog is much more important: You can't plant things in a fog and it could cover whole screen sometimes. I think lantern will be more expensive so you have to use some fire plants for that... but they will be extinguished during the rain. But we still have lightingreed, right?
Blover and hurrikane could blow away fog for several seconds. Cherry bomb, Jalapenio could help you to get rid off of mist.
2 extra Zombies: lamplighter. He is fast and could move to another line if there's a light source. Then he turn it off.
And the hound of the baskervilles. This zombie dog is stong, fast, he could move to any line, go backwards, he insta-kill plants... but he can do that only in fog and he never will leave it.
Also have a small pic of sleuth zombie. I didn't want to make it but I hate when people ideas is just a bunch of words. Also my English is bad so I hope It wasn't so painful to read.