Susan?! Good grief, she made out with the Doctor, and you're suggesting she's his granddaughter
And it doesn't make sense to assume she's just regenerated, because that's a completely different thing from what's going on here. Regeneration is one individual changing appearance and personality. This is multiple different women in multiple different eras having the same appearance and personality -- and even the same name, or nearly so. That seems like some kind of temporal echo or hiccup or something going on.
Remember how Captain Jack is "a fixed point in time" and thus immortal? Maybe Clara is something similar, but more of a recurring loop or resonance in time.
Here's a cockamamie idea I thought of which is probably bogus: The name "Clara" means "clear, bright" and "Oswald" means "divine ruler." Bright divine ruler = White Guardian? Could she be some sort of avatar of a cosmic force? Hey, if Moffat brought back the Great Intelligence...