Interesting post!

I don't think you can directly interpret how good a card is based on its win rate. Or even conclude that cards with a high win rate are causing the high win rates. It might be that a card is expensive and the ability to play it correlates with having an expensive collection. Therefore, a high win rate might just mean that the most fierce players own that card at max level.

The heatmap plots look cool. Did you make them in seaborn / matplotlib? Is the win percentage referring to the win rate of the summoner on the y-axis or x-axis?