Irelia passive and R are her biggest design problems
and imo the reason she will consistently be a pro/high elo problem until addressed.
Her passive is several modest-to-strong steroids that starts off valuable, scales excellently, and is on for the entire period of any fight she takes. She also gets multiple stacks for ragging multiple enemies with her passive, unlike, say, Darius who has to hit the same person 5 times. Why this ends up being an issue is that Irelia is very good at continuing a fight, and especially in the mid lane but also in favorable top lane matchups, she can come off as almost unfairly good at just running you down if you don't zone yourself off the wave after she starts stacking up minions, on top of how well her kit scales.
Second, her ultimate is just never a tool that is bad. A while back they removed the disarm mechanic and then buffed its damage and slow (to 90%) and I think they should have done the opposite. This very generic and effective tool is IMO a huge contribution to her everlasting pro presence because its almost always valuable and its value is never weakened by what the enemy team has picked.
Lastly, her W; I see complaints from time to time that her W feels like a bit of a dead spell, and its only useful for getting your stacks up. I've played a little bit of her myself and I can say, I sort of understand that sentiment.
I feel like her ultimate is the easiest to fix; enhancing her niche and giving her a stronger and clearer identity as the antiADC backline diver, and cutting out the fat of generic power in her R for returning its disarm, is the best choice.
I believe that the best way to fix her passive is changing it back into something like her old Hiten Style; where she has a simple button tap, a cool down around it, and she plays to that strength. You can add a condition to that effect where she still has to work for optimized damage beforehand, but having a clear steroid downtime is definitely something that would make her design healthier. I think it would also make her more accessible, which would help bridge soloQ and pro performance if she's easier to pick up.
What catches me on this proposal is that I don't know what her passive should be if I had my way on this. But I think this is the right track.