Items that have stacking passives can now stack indefinitely. e.g. RoA will now stack health, mana, and AP indefinitely, not just to the 10 stacks. Tear will stack mana with no cap Guinsoo's Rageblade goes beyond the 8 stacks and AS is capped by the current 2.5 cap Bloodthirster's shield has no upper limit Even Sword of the Occult, Mejai's, and Rengar's Bonetooth Necklace have no limit
Other than Ryze, who would become OP?
Master Yi and Tryndamere (not that they aren't already OP). Give them tons of Attack Damage and Attack Speed... Then throw in a Bloodthirster.
Also, truthfully most mages in general would become OP. Yes I understand Ryze's abilities scale with maximum mana, but remember that Seraph's Embrace 
not only boosts Ability Power by 3% of maximum mana (dear freakin' Christ, Veigar...), but also has the active which, and I quote, "Consumes 20% of current mana to grant a shield for 3 seconds that absorbs damage equal to 150 plus the amount of mana consumed". With a high enough maximum mana, any mage with a full mana bar could walk into the entire enemy team, activate this item, and last through the whole shield and still not have it destroyed. Sure that would be an overwhelmingly high amount of mana needed, but still.
seeker's armguard would make mages really tanky if they buy it early and farm a lot
