thinking of doing one and I got 3 Ideas for it:
Idea 1
Each Character aka plant cant level up with you getting new and stronger plants(though each one is unique enough to still have use) and zombies are balanced with each world going in difficulty up a bit.
Idea 2
Plants can be leveled up with the use of in-game currency and pinatas(chests if you dint play the state-of-art classic) and zombies get thougher and plants get stronger in each world with you having to level the plants up if you like a particular ones play style and abilities.
Idea 3
A mix of both

Also I will try to make the graphics of players house in the time the poll takes place.