I really, really hope Tashigi is not Kuina who lost her memories.
That will be one of the biggest disappointements ever.
I also believe Kuina didn't die falling off the stairs. Imagine beating up Zoro 2000 times and dying to stairs. Some things don't add up.
Kuina is such a wildcard, it scares me.
I could easily see your worst fear happening.
Kuina fell, and had a traumatic head injury, to the extent she appeared dead. However, she eventually awakened, but suffered a severe head injury, and damaged her eyesight as well. Her father Koushirou did everything he could, but there was no medical cure. So, knowing it would break Zoro's heart to know she could never fulfill his their promise, he had Kuina sent away to study swords themselves, thinking she could no longer fight due to her injuries. But, Kuina kept working hard, and became a swordsman anyway.
Kuina needs a huge trigger to remind her of her past with Zoro... like ZKK?!?!?