For the longest time, League of Legends has suffered from the unintended itemization issues where an item that originally wasn't meant to a specific character / class suddenly becomes extremely overpowered in the hands of that champion / class. Strongest examples of this were seen with Lethality, first iteration of Duskblade, various iterations of Black Cleaver.
In the situation where items become overpowered on a specific champions that they weren't made for, often times we would see the nerfs given to champions rather than items before nerfing the items themselves leaving nerfed champions in the dust for the remainder of the Season.
This issue arises simply because of the fact that every champion in the game has access to every item in the shop. You don't have to dig deep to understand that it could create massive problems in terms of champion balance.
A proposal on how to contain items to the characters they're meant to be used on is introduction of class items. For example, imagine if only Assassin class champions can buy Duskblade, Ghostblade and Edge of Night. Only ADC's can buy Zeal items, IE, Stormrazor and Essence Reaver. Only Fighters can buy Guinsoos, Steraks, Black Cleaver and so on. This could resolve an issue where a broken build pops up that absolutely destroys the balance of the game which in turn means that a champion has to be nerfed which in turn means that the specific champion gets absolutely dumpstered unless he follows the specific build.
In addition, tying items to classes could allow Riot to balance outliers (OP's, UP's) in the specific champion class really easily - is every champion in the class fine but only Zed is smashing everything? Nerf Zed. Is every Assassin smashing everything? Nerf the items.
Now, of course not every item has to be tied down to a specific class but if an item is specifically designed as an Assassin item, perhaps it would be wise to tie it down to that class so that no one else can use (abuse) it.
Anyway, that's just brainstorming.