I would like him to be fast (maybe more than the fighter) and to be balanced in melee/ranged attacks, about the light armor, as I recall KA has the same armor balance for all the classes into the game so I think there won't be any changes (please correct me if I'm wrong), we could make the spell work like some kind of special attack maybe...
About the character not fitting into the game, there's a chance that this could happen, but of course, we are speaking of a selectable character, so those players who don't like the new class can choose from the other classes, so I don't think that would be problem at all,
also, we need to work on the character feel and playability for KA, I also think that the game needs to have some totally new characters, that's why I felt right for making this.
I think our only choice would be to make it and wait for user feedback on it, if there's anything we can make to change it we can try it (remember that anything can be changed between releases), I think there are going to be a lot of opinions on this into the team and outside of it, so all we can do is to try it, as there's no other way to know if it will work or not and I think it would be good to give it a try.
About the balance, I also think it is going to be difficult to do, but we still have time to work on it, and as I have mentioned, we still need a lot of feedback on the project from the user, so I repeat, the only option we have is to try to implement this or any other things that we decide to implement, because there won't be another way to "test" and decide if it works or not.
We can still discuss this and come to another option maybe.