Isn't the flaw in Callandor its inability to buffer the wielder from drawing too much power?

If so, then I suspect that the flaw can somehow be used against the DO. Perhaps it can be put in the hands of Ishy/Moridin (as suggested by others) and they can draw so much of the True Power that they are destroyed and the True Power (which is the Dark One's power) is completely dissipated. (I don't do much theorizing about the OP and the TP, but I suspect that there's a yin yang thing going on such that the DO and the TP cannot be destroyed. That would destroy the balance that makes up the universe. That said, I thought that RJ promised us an end with finality, not just an end that leads to another beginning. Could be wrong about that though.)

Or perhaps Rand can use it to channel the TP to some end, like using the TP to again trap the DO.

For years I suspected that Callandor was just a symbol, just a sa'angreal that wasn't nearly as cool as the access key/awesome sa'angreals buried in Cairhhien (sp?) and on the isle of the Sea Folk. I think we've been given a hint that it is so much more, and we probably should have suspected that, given that seizing it from the floor of the Stone was one of the signs that the Dragon has been reborn.