I think Water branch is honestly fine now (minus that random wakfu spell), but if it was going to be changed, I think having a built in healing mechanic for their self or allies or both would be interesting. Perhaps their spells could apply a rejuvenating buff to allies and the panda casting that gets better as it stacks on them (like, being healed for a certain amount on their turn), while having the opposite effect on enemies. Maybe every 10 of this buff/debuff raises and lowers final resist by 1%. Giving allies 10% final resist at 100 buff, and enemies negative 10% final resist on 100 debuff. I think that would be very interesting and useful.
For fire, I think if the debuffs were actually good and somewhat consistent, without making the damage pointless, that it could be as what others requested, a debuff fighter. Able to put out acceptable damage with the added benefit of weakening the target's defense and ability to counter offense. Maybe -% damage, heavily -ranged, losing ap/mp, canceling out positive buffs while keeping negative ones, maybe a spell that heavily lowers their ability to be healed. I think a debuff that causes madness by a solid chance would be interesting. I never see it in action and think it deserves more use. It would be very enjoyable seeing an enemy misfire and hit allies or do stupid things every now and then. x3
For earth, I'd like to see this remain a burst damage option. Just about every branch has an option for all out damage, and earth seems built to do this as it forces the panda to hold their barrel to use its spells, making it much more difficult to use specialties. There should be some sort of buff mechanic built in and special abilities to the spells should remain to make up for the difficulty it will have using the class specialties. With that said, a passive specialty to reinforce this role probably wouldn't be a bad idea. The only off-tanking ability that the earth panda needs is the lock one. Every other spell should be dedicated to the role of doing damage and raising the status of their buff. The special effects of the other spells would best be utilized to complement the burst property of the class. Having those special effects be magnified through crits and making the built-in buff help with crits might be an interesting twist.