Evolution mechanics

Right now evolution just feels like a means to an end. It doesn't make the game feel any better, as it is if we had seed slots like normal the game wouldn't really change. Here's a handful I would find fun- I think they could vary from level to level, helping to make each one more unique and memorable.

Star fuel- star fuel (or whatever you want to call it. I wanted to make it like space rocket related since evolution is tied to Meteor Z) unlocks evolution through stars. At 0 stars, you can't evolve your plants at all. Each star unlocks a new tier. This is a snowball mechanic- kill zombies faster, unlock your stronger plants, kill zombies even faster, repeat. If you lose in a level with this mechanic, you can change to a more aggressive strategy or playstyle, meaning players can actually learn something from failing in levels with this enabled. 

Acid Rain (rework)- while the acid rain is active, plants that arent at full health can't evolve and also take damage over time based on their missing health as a result of their open wounds. Every wave, the acid rain intensifies, dealing damage to plants regardless of if they are already damaged or not. Acid plants would be immune to effects of intense acid rain.

Poor soil- poor soil appears as patches of 3x3 or 5x5 areas on the lawn. Unlike sod, poor soil can be planted on by anything. However, a patch of poor soil can only support a certain amount of sun. If you try to merge or plant something that goes over that limit, it will not let you. However, this only applies to rooted plants- sunflowers, peashooters, etc. What this doesnt effect is iceberg lettuce, tangle kelp, spikeweed, wallnut, sun shroom, and lime bomb. Other plants it wouldn't effect that arent in the game would include squash, rotobaga, and lily pad, just to name a few. It would also have a different appearance in each region, providing a well adjusted visual break to each lawn it graces- or well, disgraces.

Anti-merge mechanics
Just anything that can punish merging in general. Especially with how absurd sun generation is in this game (you can end levels with thousands of sun just by making a single sun tower) just anything to maybe make players consider holding back a little. This could be anything from a proper frail threat that could overwhelm the player with the reimplemented wave health system, or something that directly attacks merged plants better. The one I made a while ago, magician zombie, chooses a tier 1 you brought and makes plants explode based on their max health and usage. For example, if a wall nut is drawn, wall nuts explode for 8% of their max health, also dealing that damage in a 3x3 area, while juggernuts would explode for 32% of their max health in a 3x3 area. This would only be in the magician's lane, though. 

Pro-merge mechanics
Likewise, anything that would encourage players to merge would be nice and introduce fun niches to plants like pult shroom and strikehorn. For example, saganaki blasts could also block lobbed shots from lower tier merges depending on their health stage. At full health, only tier 3 lobbers can shoot over, at the first degrade, tier 2 and 3 can, and at the second, all can. Then they could introduce other hazards in other area- like pipes in the sewers

Proper fodder tier 1s
The two I've put in my fanmade roster, puff shroom and button shroom, essentially promote a plant to tier 2, similar to sun shroom right now, but with no new mechanics and usually downsides. For example, scaredy shroom gains the hiding mechanic, while having the stats of essentially a tier 2 peashooter to compensate. These plants allow players to get valuable plants without needing to wait for cooldowns or spending a ton on other plants. I could also see iceberg lettuce and grave buster being "fodder" plants as well. The cost behind them is the seed slot, their recharge time, and the fact you lock yourself out of other merges, as well as a compromise in overall strength. Cheap fodder plants have always existed in pvz and I really think they would be fun ways to add options to make new merges that give players things to consider other than just sun cost.