I think this is a really hard decision to cut down to one on, but I will try.
Option 1: AP Fighter penetration item somewhat similar to Black Cleaver. AP Fighters have a tough time being balanced because of the lack of good itemization they have. I think they need more than one item, but a penetration item like Black Cleaver would help drastically.
Option 2: Teleport. Either the removal or changing of Teleport from a summoner to a purchasable item. Teleport offers way too much currently and that is why we see consistently in pro games 3 players per team taking it.
Option 3: Fixing the jungle. Camps are down for far too long making the only option for junglers to Gank meaning that we have this nonstop fighting meta. It is getting old and the difference between a good and decent jungler is far more impactful on the game than the a minor difference between any other role. Junglers don't usually win the game, but lose the game.
Option 4: Removal of Stopwatch. No more is needed to be said about this trash item.
This is a really hard choice for me, but I think I would go with Option 1. This is because without a change to the itemization AP Fighters will always have issues. Either with having overly compensated kits to deal with the itemization problems or just being countered by 1 MR item.
I would like to hear what others think. Let me know what you would want changed if you could choose one thing.