@GutmanSca I upvoted the post because it is very detailed, which I love and has some great ideas (although I disagree with some). Overall, a very nice well thought out list from you and your cousin. Now to my opinions on the points:
I don't really like the changes to his weapon. Damage drop off most likely won't come, even if we keep reiterating we want it. Therefore we should not consider it when discussing balancing, in my opinion. As mentioned before, his primary wasn't significantly nerfed. I think 6,5 damage is just too much. His rate of fire was already increased and he is designed for bunny hopping, making him hard to it with the small hitbox. Even now in his nerfed state many players get top vanquishes with him because of bunny hopping and the mech actually still being useful, despite the general opinion on it . I continue to get to the top of the leaderboards with him. Therefore, I suggest a compromise. 5 - 5,3 damage would be good.
I know you other changes like the gravity change and ammo capacity reduction try to balance this, but it wouldn't help much, because even with 40 ammo he would be able to quickly vanquish any plant with the much increased damage.
The gravity grenade canceling more abilities seems good, although I somewhat disagree with Acorn Dash. It is meant tor dodging and escaping. Again, you could say the grenade is meant to catch targets. So I don't know what to do, since bot abilities core concept cancels with the other.
Mech:
Giving mech armor would be good in my opinion, but disable healers for him at the same time. In nearly all lobbies I play there is at least one healer glued to Mech as soon as he is summoned, if not multiple. This completely destroys balancing. In addition to him being heal, he can be overhealed! This simply shouldn't be possible.
I also don't like the damage increase per bullet. 3 increase is too much. I would like his primary to deal 6 damage.
Don't increase Bionic Bash's damage. It already is powerful and useful for insta-vanquishing low health classes, like Nightcap trying to fungh-fu Mech.
In my opinion Wizard shouldn't be able to attach himself to z-Mech. The mech is supposed to be a powerhouse, and adding a damage focused "support" on top is terrible.
Missile Madness:
I would like them to make the damage dealing area a bit wider, but the missiles in turn deal less damage. They could add the zoom feature to the ability. This way, you could coose between dealing less damage to more plants, or more damage to less plants.
I really like the changes to the upgrades.
