Maybe a few should not even be able to ride a mount, but get other bonuses instead (to compensate)
Anyways, I don't think we can really go wrong in the realm of champion differentiation. If they are all blank slates, they are completely customizable ... and if they have ENORMOUS branching trait trees, I really don't think more complexity in trait decisions can ever go wrong in this one particular instance (as long as we only have a few choices at each particular junction, but they take us down a specific path-set based upon our class, and our early decisions.
Also, going the other way, if they all already start with their own Class, legacy, unique spells, unique abilities, and we can only choose between a few path-sets (He is Zealot Warrior of Aragon, you can choose either Divine Crusader, Soldier of Light, or Rage of God (Bezerker type) subclasses ... and each would have a modicum of branching trait paths.
Zealot of Aragon -> Divine Crusader, Soldier of Light, Rage of God
Warrior of the North -> Blood Bezerker, Soldier of Ice, Barbarian Lord
Barbarian -> Blood Bezerker, Conqueror, Barbarian Lord
Eastern Dervish -> Sand Bezerker, Soldier of Stone, Prince of Thieves
Halfling of the Hills -> Soldier of Stone, Soldier of Light, Prince of Thieves
Gared the Rogueling -> Prince of Thieves, Master Assassin, Bow Master
Frank the Archer -> Bow Master, Soldier of Wind, Flurry Artist
Assassin of the East -> Blade Master, Shadow Arts (assassin/mage combo class with shadow magic and ninjutsu abilities), Master Assassin
Assassin of the South -> Fire Dancer (acrobat/mage combo with free weapon enchantments), Bandit Lord, Master Assassin
Assassin of the West -> Marksman, Master Assassin, Shadow Dancer (assassin/acrobat combo with stealth abilities)
Sharpshooter -> Marksman (better criticals with ranged, higher crit chance), Bow Master(stronger range attacks, various types of "arrow" abilities"), Elite Sharpshooter (100% accuracy w/ ranged ... chances to ignore all armor)