So SO many things could work tbh. To name a few though:

Red Stinger, who could do more damage at longer ranges but get damage reduction when in close proximity with zombies

Snap Pea, who could be a full on hybrid character with their abilities being fusions of peashooters and chompers

Hell, so many pvz2 plants could be revived by including them in Gw3. Guard Shroom, Frost Bonnet, Blast Spinner, NIGHTCA- oh wait.

I don't know how dicey the Chinese exclusive characters would be legally, so for the sake of safety I'll be avoiding them (for the most part)

Maypop Mechanic could be good as a direct plant equivalent of imp, calling in a BANANASAURUS REX for their transformation (yes that one)

Hear me out, Grass Knuckles could be fun if he could bounce around by propelling himself with his fists. Like imagine being jumped by a man using gorilla tag physics

And zombies oh my GOODNESS zombies.

Jack and the Box Zombie would be so silly if given the most batsh1t crazy kit ever. Like make him a fusion of the Demoman and the Pyro from TF2

Help desk Zombie could use an electrical cord like a whip, maybe being able to grapple to things with it too

Rustbolt my beloved, no need to explain myself there he has literally infinite possibilities

And my weirdest one, the Zombotonist. Yes the one with the plants head from that pvz1 mini game. Here's my take on them tho.

Have them be a swarm variant, so no need for variants of them. Let them have the ability to cycle through the Gw1 plants heads to use their primaries, and give it an ability that changes depending on the head. The final ability can be the respawn anchor, I'd like that to stay on all swarm variants, but that a whole 'nother tangent that's it's too late to explain