Finally, This is my third part of the Book'o'Ideas! I definetly hope that you will use some of them!
Starting off, with some extremly basic plants:
1. Tomato - Cost: 175 Sun - Tomatos are a really good plant, but some people think they are not very useful. Tomatos spit some kind of goo in 4 tiles ahead of him, that if zombies step on it, it damages them slowly with 1 normal damage. It's like a Sap Fling copy, but in a different way. Plant Food does the same as Sap Fling. The recharge is slow.
2. Apple Bomb - Cost: 50 Sun - Apple Bombs explode if tapped on it, Damaging zombies in a 3x3 radius cross shape. Plant Food will make him create two copies of himself on a different tiles. The recharge is slow.
3. Stompear - Cost: 150 Sun - Basically, it's a smarter version of Squash, that regrows itself after squashing zombies. The growing time is like 20 seconds, and the Recharge of this plant is slower than Squash. Plant Food will make him launch 3 explosive pears in the air, that will explode on a random tile with a 3x3 splash damage radius, dealing a massive damage. Deadly to Bucketheads. Pun between Somp and Pear.
4. Cypress - Cost: 175 Sun - Cypress' burn zombies with the glowing eyes of this tree related plant. It takes 5 seconds to turn a zombie into ashes with this plant's glowing eye effects. Plant Food will make him basically act like the Laser Bean's plant food ability. The recharge is sluggish, and the special thing about this plant is hides if a zombie is close.
5. Frizbloom - Cost: 175 Sun - Basically a slower recharge Bloomerang, that shoots frizbees, that go trough 4 targets this time. Plant food effect will make him shoot three giant frizbees on 3 random lanes that has the most zombies on it. Special, becomes a defensive plant if a zombie is close.
6. Charm Shroom - Cost: 200 Sun - Charm Shrooms blow kisses to zombies every 10 seconds. Its deadly to Coneheads. Plant food will make her to blow a giant kiss up the sky, that if reaches the ground, explodes and defeats literarry every target, exept Gargantuars on screen.
Sorry, Couldnt give more ideas. I stole theese plant names by the unused ones from PvZ: All Stars! Hopefully, you use them!