If you really want to make the game more challenging there needs to be limitations on how many mythic and rare cards can be in a deck.
The limit should be something like 2 mythic and 3 or 4 rares per deck. It would be even better if cards were limited per tier, per color, per deck, that way each planeswalker deck you have is limited by your mastery in that planeswalker’s color.
You could do something like:
Bronze: 1 mythic and 1 rare
Silver:. 1 mythic and 2 rare
Gold:. 2 mythic and 2 rare
Platinum:. 2 mythic and 3 rare