I voted for the combo XD/spells option.
Reason being... the mechanics can be described any way you want.
People want the Paladin to have a Smite ability. Okay. What does that entail? Presumably it means the Paladin will do extra damage when he hits with his weapon, and that damage is defined as divinely magical in nature-- quite possibly with extra "stuff" on top of it, like it's Holy damage or something.
How exactly is that different than the extra damage the Fighter gets with Expertise Dice? Except for the barest of additional mechanical advantage of the damage type keyword (whatever that ends up actually giving you)... all the other bits are just Story. The Paladin does additional damage via "divine magic" instead of "additional martial skill". But the actual mechanical effect is the same... the Paladin can occasionally do more damage on a weapon hit, just like a Fighter can.
So I don't really see the need to create a "new" Smite mechanic, when the actual results are no different than what we'd get with Expertise Dice-- additional damage. So long as the Story and fluff layered on top of the mechanic is different... and you play the game such that that Story actually matters to the fiction you are creating with your game... then I think we're good.
And as an aside to SageMinerve's assertion to lower the martial capabilities of the Cleric to distinguish it from the Paladin... I think that hit die and weapon and armor proficiencies (especially via domain/deity selection) will more than accomplish the requirement. A Trickster or Sun cleric will be quite less melee capable than a Pally by design. The Justice or Storm cleric (ie the melee-focused ones)? Those I don't mind if they start approaching the Paladin, as that is what their deities want their clerics to be.
Reason being... the mechanics can be described any way you want.
People want the Paladin to have a Smite ability. Okay. What does that entail? Presumably it means the Paladin will do extra damage when he hits with his weapon, and that damage is defined as divinely magical in nature-- quite possibly with extra "stuff" on top of it, like it's Holy damage or something.
How exactly is that different than the extra damage the Fighter gets with Expertise Dice? Except for the barest of additional mechanical advantage of the damage type keyword (whatever that ends up actually giving you)... all the other bits are just Story. The Paladin does additional damage via "divine magic" instead of "additional martial skill". But the actual mechanical effect is the same... the Paladin can occasionally do more damage on a weapon hit, just like a Fighter can.
So I don't really see the need to create a "new" Smite mechanic, when the actual results are no different than what we'd get with Expertise Dice-- additional damage. So long as the Story and fluff layered on top of the mechanic is different... and you play the game such that that Story actually matters to the fiction you are creating with your game... then I think we're good.
And as an aside to SageMinerve's assertion to lower the martial capabilities of the Cleric to distinguish it from the Paladin... I think that hit die and weapon and armor proficiencies (especially via domain/deity selection) will more than accomplish the requirement. A Trickster or Sun cleric will be quite less melee capable than a Pally by design. The Justice or Storm cleric (ie the melee-focused ones)? Those I don't mind if they start approaching the Paladin, as that is what their deities want their clerics to be.