Mechanics I will shed some light on right now. 

I was thinking(if there's overwhelming negative reactions to this, then I will abandon it) of having a sort of point system in play. You will of course choose your Realm, and you can divide some arbitrary number of points between the two elements that fall into it(Ascendants will have an arbitrary number of points fewer than the other two classes, because of aforementioned necessity of an anchor element). This will represent how much of the "element crystals" you have. 

I also thought about building literal skill trees and assigning points as the RP goes on. If people are open to that idea, I would love to implement it. I'm actually very fond of skill trees. 

Whether I do that or not, these are the kinds of powers the various classes would use(in addition to the literal interpretations of their elements):

Knight: buffs, and blunt force attacks, basically. Healing would actually fall in their domain as well. 

Monk: Gliding and possibly upgrading to full fledged flight. Possibly transfiguration. They can meditate and connect to the minds of other meditating Monks. Monks are actually not ruled by any single person like the other two groups, they instead operate off of a collective thoughtfield. 

Ascendant: Teleporting, bringing objects into existence(simple objects, no biological or machines) and making objects disappear(same caveat). 

Common cultural behaviors. There are outliers, and remember that there are still ways to use these traits in a way that makes the character your own. 

Knights: the obvious prideful sort of things. A lot like American soldiers, just like the Marines think they're the elite branch, the Knights consider themselves the backbone of the Heroes Consortium. 

Monks: Monk doctrine encourages the use of both of their elements equally and in balance, but they have a surprising amount of outliers in either direction. Though some fall in with the doctrine, a lot of youngsters find it stifling and rebel in small ways. There are probably more single element users in the ranks of the Monks than in either of the other two. 

Ascendant: Detached from reality, and easily distracted. Especially as their affinity increases. On the flip side, a lot of them try to find ways to counteract this side-effect of their Realm.