I've noticed the biggest frustration with TFT is the RNG, and a lot of it is the champion pool. Getting the wrong items for your comp sucks, but a skilled player can pivot to something that works with them. Something that can't be avoided, though, is getting played by the rerolls.
You've been given nothing but large rods and recurve bows, but the shop insists you grab several wardens, repeatedly putting them in the shop, and never giving something useful like BMs or rangers.
I'd like to suggest a sort of RNG Player Protection. Something along the lines of reducing the odds of getting a champion if, after X rerolls, Y champion has appeared Z times without being bought, the odds of it appearing in subsequent rolls is reduced by N%.
Obviously this should never put a champ at 0% chance to appear, but right now it's too dangerous to roll unless you're at the point where adding additional champs to your board is pointless and you just need upgrades, so usually level 7, 8, or 9. It would reward players for playing well and econ'ing enough gold to roll for exactly what they want, and would reduce frequency of the frustrating feeling of being in 8th place, looking for that ONE champ that could save your comp, and never drawing it.