Rumple and the audience are assuming that he’ll lose all of his powers with True Love’s Kiss, but what if that isn’t necessarily the case? Another possibility is that instead of destroying the darkness and his powers, it converts the darkness into light. After all, we know that the Dark One’s power is sourced from the Holy Grail just like Merlin’s. It was not darkness originally, but became darkness once Nimue crushed Vortigan’s heart and tainted her magic. So maybe Rumple would become the Light One, as shown in Season 4’s finale, and this could even “cleave” him from the dagger. The Dark One willingly ready to lose his powers to save their loved one could be the ultimate test of selflessness.
I would’t mind this outcome, I think, so long as Rumple doesn’t revert to power-hunger/dagger loving in S6.