Leveling a plant should also level up its tier 2 and tier 3 plants.

What I mean is, instead of having a Peashooter at level 7 and having a repeater at level 2. If you upgrade a Peashooter, it would upgrade all the base fusions for the plant. So all of them would be level 7. When it comes to a mix of two plants, combine the average of two plant leveling or something to make an actual fusions level. 

Thank you for reading this, and have a great day!