make a 5mana 5-6 minion for druid, with this description.
Star of Game: If your deck has no Neutral cards, upgrade your Hero Power.
Hero power type: 0m passive.
Upgraded power description: If you end your turn with any unspent mana, add a coin to your hand for each mana not spent.
I was thinking you could do nothing for a few turns then pump out some big minions, either way I had to post this idea when it came to me hope the community finds the concept entertaining