First off I just want to say that I appreciate the mod and all the hard work and dedication you guys have put in to it!! This mod, and ones like it just add a whole lot more longevity to the game and it's absolutely awesome to see!
Anyway, I think using the idea of the Shaman and Druid from WoW would make for some really cool classes. They already essentially have their own skill trees that you could follow and be interesting at the same time.
As for the Druid and shapeshifting, is it possible to reskin the character as a pet as a spell?? Which then in turn gives a buff? Or even just make an aura and symbol above and around the character to simulate a "form".
The Shaman should be centered around totems for buffs, debuffs, utility as well as adding a little bit of damage. They should also have a melee dps tree and a caster tree. And be able to summon 3 spectral wolves
A spell casting class that had at least one tree focused primarily on DoTs I think would also be pretty damn sweet. Being able to group up a bunch of mobs, throw some DoTs, and kite them till they all drop dead!
Just thought I would share my ideas, anyway.