Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Divine Champion must be able to get Great Charisma as epic bonus feat and Extra Smiting feat as bonus pre-epic feat.
Because (Great Charisma); the unique ability of Divine Champion is Divine Wrath, Lay on Hands, Smite Infidel features are charisma based.
Because (Extra Smiting); The Extra Smiting feat's requirements include the Smite Infidel feat.

I see where you're coming from, but that'd make them way too powerful.

I agree with Louvaine. The 3by30 rule adjustment already has given DC a huge boost as you can take most if not all of their levels in epics and get a ridiculous amount of epic feats. Opening up great charisma would make them extremely OP.

Let's say Great Charisma makes the class OP. What about Extra Smiting? Is it too?

Probably not, no. But agreed with the above, that's a lot of post level 20 CHA bonuses. Five alone, with DC bonus feats.