I have something in mind for a long time about this. And it is about making every role separate from the rest and items intending to be used by a specific role be more effective to that role and inefficiant for everyone else. How this works is simple
Take each role and depending what it's intended strengths are, boost the appropriate base stats while at the same time significantly reducing the rest.
For example, what's the primary role for an Assassin? Bursting someone down quickly and either get out if they can or die with them. What stats work for that? AD or AP. So, boost their Base AD significantly if they are AD and for AP it works differently. At the same time, reduce their defences and HP pool.
A tank is supposed to have defences and soak up damage while doing some aoe and have some cc to soften up targets for their carries and disrupt the nemy team. They need defences and survivability. So, what do you do for them? Bost their defensive stats and HP pool and redutheir offencive stats, aka AD by a significant amount.
For mages, and AP assassins, their role is to either burst someone down and kill them fast or control the battlefield with a lot of CC and AOE. what do they need for that? AP and some more control oriented also need survivability. So, they get more AP and moderate base stats, higher than an assassin but lower than a tank.
A bruiser is meant to disrupt the enemy team and lock down a target so that they take them out or cooperate with their carries/assassins to do so. They are tough and pack a punch but also, have to pick their fights carefully, they are not unkillable. An assassin should be able to kill them and either escape or die with them, an ADC should be able to kill them by kiting properly but, if either assassins or ADCs stay too close for too long, they will die too fast to do any significant damage. They get moderate defences and offensive stats, each tuned individualy based on their kit and lore to suit the character. Bruisers are a hybrid between a fighter and a tank anyway
Fighters are essentialy melee carries. High damage, moderate to low survivability and some defensive mechanics, Trynd's ult, Yasuo's W and passive, Fiora's W etc. This class gets high offensive stats but moderate to low defensive stats, based on their innate defensive mechanics. That's high base AD but moderate to low armor and mr.
A support is a tanky champion with a lot of utility and lockdown meant to support his carries, ALL OF HIS TEAMMATES, NOT JUST THE ADC, that's for those adc players that want their support exclusively for them... grow up guys. So, they get high defenses and HP.
And finally, our beloved and hated ADCs... These obnoxious champions that only know how to right-click and move around.
They are as fragile as the assassins but with almost equaly strong attacking capabilities. I say almost equaly because Assassins are casters whiled most marksmen are AA based and don't do nearly as much damage as an Assassin with their abilities. Like assassins, they should have HIGH base AD and low armor, mr and hp.
So, now that we got that out of the way, let's talk itemization.
All items shall give stats based on the BASE stats of the bearer, Critical chance being the only exception, none has individual crit chance alone and it should remain this way. They also have other passives that work independently but that's it.
So, each item will provide it's stats as a % bonus of the bearer base stat. So, a bloodthirster will give X% Base AD and it's passive that works independently is it's lifesteal. Same goes for Ravenous Hydra, it has it's lifesteal as a stat, well, since there is no base stat lifesteal, this stat will become a passive for the item.
For an armor item, let's say, thornmail, it gives X% Base armor. And an example for hp items, a Warmog's armor will give X% of your base hp pool and of course, as you lvl up, it will also give more hp because your base hp pool increases.
For AP items, they will either stay as they are because there is no base AP stat, or Every champion that scales with AP will be given a Base AP value of 100 base AP, and all abilities will be changed to scale with bonus AP and the scalings on the items will be made so that they are the same as now. But, it's easier to just keep them as they are, it is the same thing.
Now, this way, items meant to be used by an Assassin/ADC/Melee Carry, will not be effective for a tank. Tanky items meant to be used by Tanks/bruisers and some fighters, will be inefficient for an Assassn or an ADC. Those items will also be less effective for fighters than for tanks or bruisers. And thus, there won't be Tank assassins anymore. Each role will have distinct strengths and weaknesses and the game will be a lot more balanced.
Of course, some champions will have some tweaks in their kits to compensate for those changes.
So, what do you guys think?