This looks really cool, and I'm really interested in joining. I love skill trees too. I'd be totally up for helping you make those, if you'd like. I even have some ideas.
Of course these are just suggestions, or ideas I have, so if you don't like any of them that's cool. I'm just putting them out there.
The soldiers have earth, and water, and their skills are going to deal with buffs, right? Well, what if one of their skills, or even a few of their skills, possibly in the middle tree, had to do with the manipulation of their body weight? Imagine a soldier charging through enemies, hardening his body, and increasing his body weight so that his momentum carried them forward like a truck!
Now as for wind and fire, since together they create storms, maybe the middle tree could have to do with electricity like they gain abilities that involve lightning, and can even tweak their own nervous system.