I love the idea of creating new classes (Bounty Hunter being my favourite choice).
You could create new feats for each class. Also you could create new unique items for each of the seperate classes and spread them about a bit like they have done on TSL. (For example: Kubaz scoundrel gloves, Nikto Soldier gloves.)
I also suggest adding new hides (and scripting them to be obtained by Trask in the opening level) with both positive and negative down sides to each of the classes.
I am going to copy over a small post that has already touched upon creating new classes for KOTOR:
Many thanks and good luck with your new mod.