Some thoughts:
Her Q would probably be left mostly unchanged, as it's already fun and unique, and a source of mastery.
Hiten Style would probably be kept in some form, probably retaining the sustain boost via attack aspect. This skill is already part of her lore, and would probably be amped up in the story.
Her E is probably the most likely to hit the chopping block. It doesn't allow a lot of counterplay for her enemies, and it's pretty boring for Irelia as well. It would likely be replaced by some CC substitute, possibly a conditional stun, different from what she has right now, but something she has to set up and her opponents can plan for.
Her ult could be much more interesting too. Part of her fantasy is her floating blades, and the ult is a great opportunity to do that. I don't think her current ult delivers, being little more than her waveclear tool, occasionally finishing off a kill. I'd suggest using the latter, making it a short range, single target skill shot execute to finish off enemies she can't reach, shooting a blade with the flick of her wrist for an epic finish.
Her passive is also a place to explore. I was thinking some kind of "she has a certain number of blades floating around her every X seconds", and this can be a secondary resource playing into her other abilities, like her ult and E. It gives her highs and lows she has to play around and some decisions how to spend her blades. But that's going into theory crafting territory. I just think her passive is very likely to change.
Overall, I really like Irelia's theme, and I think her kit is cool too (especially Q) but I think her theme could be played up more and her kit made a bit more interesting. In particular, her visuals could be this really cool martial arts style that flows into using her blades as extensions of herself. Right now she just looks like she's bonking people over the head with a wooden plank half the time. She has the opportunity to be really awesome, while still occupying her own space as a diver.
or 
and to catch off weak enemies. It has potential to do much more than that. Her Q is really iconic to her and her E is very interesting, but her E is probably going to go. It's not interactive and extremely oppressive against people who are behind.
and 
