Suggestion on how the level system could be changed.

This system, as it stands, doesn't work; it's more efficient to use Level 1 plants instead of merging them, so this reason alone is sufficient to make a valid change.

What could be done to make this fit into the game?

Well, we could use something we already have in the game: the abilities; Those that we unlock when we level up the plant to 5 and 15.

They work in the best possible way for the fusion mechanic; if you have the Tier 1 plant's ability, its evolutions will also have it.

So the changes that would be made would be: Plants would have 5 levels; since there are few levels, they would obviously be more expensive than the current ones. Each level would unlock a new ability for the plant that its fusions could inherit. These abilities would have individual levels, and I will explain why.

It would be difficult to create new abilityes for each fusion, and it would be bad if some mixed fusions repeated those skills, making it pointless to level up some mixed evolutions. So, each skill would have its own level. Here's a practical example of how it would work: I have a level 2 Peashoter, at this level, it gains the "rapid fire" skill, which makes it attack again every 9 shots, i will fuse them into a Repeater, The level 1 Repeater inherits the skill from the two level 2 Peashoters, thus giving it the "rapid fire" skill at level 2. It would be something like, "rapid fire" level 2, attacks again every 7 shots.

This would make the leveling system healthier, since increasing stats destroys the entire game's balance. Obviously, the skills that the plants have would be changed and new ones added, maybe even creating a selection of skills to create a build? I don't know, but I don't want to elaborate further here.

How would the zombies be? Since it's not enough for the plants to just get stronger, it would be too easy. Well, the leveling system for them would no longer exist. I would bring back the modifiers, but in a different way. Instead of something that increases their life, I would do something different, like the zombies moving faster in this phase, the waves starting earlier, and exploding when they die causing damage, things like that, you know? Something that might be a special zombie gimmick, or something that changes the course of how you will behave in the level, But of course, that doesn't rule out the possibility of a zombie causing damage when dies.

Regarding the resources we've already used, we could have a full refund and convert the current amount into something more proportional to the cost of the new levels.

Anyway, since I've put all the ideas in the suggestion, I wanted to consider one last thing here: I would create a level cap for the story mode. I don't think it's cool to force you to level up plants to unlock adventure mode, because its focus is the story, unlocking plants, and exploring new maps (that's my pov).

In short, that's the suggestion, the game doesn't fit with the current leveling system; it negates the game's mechanics, even if you improve the rewards, the essence of the system doesn't fit.

Please excuse any grammatical errors in the text, English is not my native language.