Hey, I've been playing with Kleptomancy and it's a great rune for supports, but I can't buy more than 1 or 2 items if I want to use it to its full potential. Plus, sometimes getting the right item at the right time can be game changing, and I think that's too much RNG. For instance, an Oracle Extract vs an Evelynn, a Stealth Ward at just the right time, a farsight alteration when the enemy team is doing drake, etc.
An easy solution for this would be either instead of getting a random item, you can buy one at the store with stolen coins, kind of like summon spell tokens. You choose what you get and your inventory doesn't get crowded. Another solution in case you like the RNG for some reason would be claiming your items at shop, so you can choose which you want to keep and which you want to sell.