Possibly the only way to add a new class would be to add it as a culture pack. What this means is that the class is identical OR almost identical to another class, but just looks different.
I have thought over this for quite some time, and it’s something I definitely think would give the game a bit more spice.
Pirates - Musketeers. Both use light weaponry, equipping the Musketeer with varied thrusting style weapons as primaries, and for secondaries, choices of bombs, a main gauche to duel wield, or a rifle/matchlock pistol.
Ninjas- Aztec Eagle Warrior. Both are nimble, and could perform rolls well, however there may be differences in their weapons as the Jaguar Warrior used the big ass obsidian club. It could be good as a glass cannon, rolling, nimble, has access to powerful weapons but not much health. Or:
Xande Warrior: Very agile, uses light weaponry, little armour. It may be a better fit but… I really… really want the Jaguar! Or:
Apache. Agile, light armour, light weaponry, rolls around a lot. Seems similar enough, and I bet a knife-fighter wouldn’t go amiss.
Spartans- Roman Centurion/Legionnaire/Marine could work, though there would be some differences as already outlined above, so I won’t repeat it, as i think it is entirely valid.
Vikings- Maori Warrior. They could share the berserking ability, and although lightly armoured, their sheer bloodthirstyness could allow them to take equal damage as a Viking. The main draw back here is the lack of shield in the Maori fighting style. So on that note:
Celts. They are already HUGELY similar. The lack of armour blah blah bloodthirstyness see above. They both use shields, and I can wager they wouldn’t mind lobbing them. They both utilised very similar weapons, spears, broadswords, clubs/axes.
Samurai- Rajput. Long reaching weapons, singular choices in ranged weapon, not too heavily armoured but sufficiently that they would likely be on par with the Samurai. And let’s face it, they both flail (Teehee)
Knight- Uhm… Uh… I’ll get back to you… Maybe. I don’t know. Maybe the Roman would be a better suit for the Knight, due to the heavy armour and general tanky-ness of the class.
Just my thoughts.