Pop Fizz: A Potions and Beast Form path have both been confirmed. The Potions Path upgrades Potion power and effect, lets you mix them and let's you hold A for all three, the Beast Form upgrades are unknown. (ON THE OBVIOUSLY SUPERIOR PATH: DON'T SHOW EVERYTHING OFF... OF COURSE!

Crusher: Probably a Hammer Path and a Projectile Path for both explosion and scream. I think his Soul Gem will be the extra Armor on a side note.
Flashwing: The tail crystal shoot and the twirl, I really don't see the chandelier getting upgraded much.
Hot Head: Hmmm... Maybe one for both cannon and rain and the other for dash... His attacks just seem... Difficult to upgrade.
Hot Dog: Meteor Shower and Fireballs. Anything else would be redundant unless Firewall isn't the Z Attack.
Bouncer: One for bullets, and one for rockets, ala Trigger Happy.
Sprocket: Jeez... Sort of hard to put it down. It depends: is the Giant Death Tank an update to the Turret, or the other way around? Because if it's the latter she's probably gonna get a projectile oriented path for both Landmines and Mobile Doom Turret and another for the Wrench. Well a Wrench Path is pretty much locked in- yes... It's all hanging on that Tank.
Eye-Brawl: He's not going to get an Eyeball Fountain path. HAH.

Fright Rider: One that upgrades the dig and the Ostritch Deathheads, while the other upgrades the Spear? A Spear Path is almost certain as in one video you see it doing 42 damage a hit which would be ludicrous outside of a specified Path, and both their other attacks seem like they could genuinely develop.
Swarm: I'm too lazy to decide for him. He just has such... Odd sets, in terms of upgrading.
Jet-Vac: I expect one for the Vacuum and one for the Jetpack. The Vacuum one would be generic projectile upgrades while Jetpack would unlock combos, etc.
Tree Rex: I can't think of ANY real upgrades for him in general...
Shroomboom: One for projectiles and one for defense, like Zook.