About the "must-have-promo-card" I understand your issue, but I think not that it is correct.
Look f.e. the Mylor (Dice-Summoner Earth) - he is definitely such a must-have-promo-card.
I think you will agree that in some rulesets he is the best summoner in the game but in other rulesets other summoner are just stronger, same with Yodin Zaku or other summoners.
So it would also be with my suggestion (and I would expect that the team will think to create a card who you need in some rulesets but don't destroy the ranked gameplay) or would would even think to use this summoner with 10-Mana-costs in a 12 Mana-battle ?