How about, instead of penalizing players for not having the 'proper' level of card- we institute a multiplier based on the level/rarity of the cards fielded?

The level 'penalty' was bad, for many, many reasons- first and foremost: it wasn't based off of 'fielded'. It was based on 'highest level of owned/rented'. So, people would own/rent cards to avoid the penalty while fielding Starter Summoners and L1 teams.
Ie. accounts would rent L5 Chaos Agents then field them as L1 because that same account didn't rent/own ANY summoners.

OTOH if there was a policy where all (non-SB) 'Silver' leveled cards got you a 4x (25x) multiplier to your earnings.... well I think you would see a MASSIVE uptick in the quality of cards fielded.