I actually have a suggestion for a perk that encourages multi color deck.
"Reduce bonus damage of players running only cards from the same element by 25%. This damage reduction effect will not cause the affected players' bonus damage to drop below the unmodified baseline."
There should be one set of cards with this perk from each element, so by running one card from each set, the perks stack and you won't be at a huge disadvantage against players with tons of viagra invested into same-element decks. Note that the damage reduction is 25%, so you only need cards with this perk from 4 element to achieve 100% bonus reduction. Then you can use whatever card for the fifth.