(Example for why this is a problem: Assassins building tank and one-shotting people. Also, Maw of the Malmortius being used midlane when it is meant to be a fighter item)
What spurred this on:
Here is an idea that could help with all item balance problems:
Reduce the flat bonuses from items significantly and add moderate bonuses that scale off of % Base statistics. Then implement a classification system for items that group items into categories based on the class it was designed for (Tank, Fighter, Marksman, etc.). Then add a game mechanic that introduces scaling bonuses that increase per category item owned (up to 5, so as to not include boots into the item scaling list). These bonuses would be unique to each classification and would function to reward players for building more of a group of items. These bonuses would primarily be increases to bonuses received from the item stats (from all categories). All champions still have access to all category items and can receive the bonuses from a category for owning that category's item.
The tank category would include a scaling increase to % of Bonus Health, Armor, and Magic Resistance received from items. Likely, the mage category would include a minor scaling increase to % of Bonus AP received from items and a larger scaling bonus to magic damage done. Likely, the marksman category would include a minor scaling increase to Bonus AD received from items and a larger scaling bonus to % Basic Attack damage. etc.
These are all just personal theories for bonuses that could be added, but the entire bonus system could be tweaked to be different. Nonetheless, I think that this is the best way to help champions specialize at their preferred build path and keep them from gaining too much of a benefit from items that were designed with another class in mind (such as assassins picking up one or two tank items and gaining full item efficiency with it, creating a state of imbalance for one reason or another). Think of it this way: In most games that include classes, many of the classes can choose items that were meant for another class, but often they do not gain the full benefit that the other class would receive with it. Buying items and building towards a desired role only makes sense, while also helping with general game balance. Implementing this mechanic would also not punish nor restrict players from building outside of their role, but instead reward those who are purchasing within their desired category while keeping someone who is not investing as heavily into that desired category from gaining the same amount of efficiency as someone who is building for that role.
I'd continue on, but I need sleep. I'd love to hear feedback on this idea and would be interested to hear whether or not this could be a viable solution to certain classes gaining too much benefit from items designed for other classes.