I feel like assassins should scale with kills

No I'm not saying rengar's passive but I'm talking more like kill GOLD, like if theyre supposed to be a snowbally champion make them EARN their snowballiness, not just farm to and with high base damage cough cough cough cough now I know most of you will say that the itemization is cheap but they should make it so that they have a different, more expensive, itemization (or just make the regular builds more expensive but that will hurt the other champs as well so idk how to do go around this problem) or make them scale, having weaker early game but better later games. if theyre suppose to 1 shot someone make them 1 shot late game and not 15 mins into the game cough cough cough cough