I've had this theory. Here's my train of thought.
1. Kuina's death is infamous for being unbelievable.
2. Mihawk is introduced early in the series, and foreshadowed as Zoro's ultimate enemy. Now, unless this fight is awesome (very likely), a twist is needed, i.e. a new final opponent.
3. Kuina seemingly had no mother. Why? Also, there's dad's line about being an average father, and his hinting to Zoro on how to cut steel, this is, can he too?
So here's a theory, Kuina's dad's wife (this guy need a name…) was killed in a tragic inncident. Maybe killed and violated (wow, that's dark) in a way that leaves an impact on this man. He seems what man will do to a women when left defenseless, and fears the same thing happens to Kuina.
Kuina wants to be a swordsman, but the father tries to discourage her. After all, no matter know strong she gets, what happens to her if she just loses one battle? Would the same thing happen again?
And then, on the night of the 2001st battle. The father learns about what happens between the two of them, and with this Kuina will never give up on her goal. Knowing that there's no way to stop her, the father snaps and kills her, as a mercy killing. The injury could be under that hankerchef at the funeral.
And after this, the father would have no problem with Zoro "taking" Kuina's dream. And from here, maybe he'll end up as the real final fight for Zoro, student vs. insane Sensei. And maybe the father, seeing Tashigi's image, snaps.
....Or we're looking to far into this and nothing happens.