First step ist lowering the drop rates like @aggroed indicated as possibility to reward early buyers.. for Legendary from 1% to 0.2% per card. But you are right the supply increases day by day and is already quite high in comparison to current holders. As long there is no tournament there is also no greater demand for the cards. For me it's about time now.. we have simply to wait until the game is out. I think many people are simply not interested just collecting the cards, they wanna play.
Another thing is, i think there are also very impatient people or addicted booster pack buyers which don't wanna buy&hold or thought of a quick buck in days but with these prices currently a seller could have a hard time. I totally agree also with @heyimsnuffles regarding the prices for Epics and Legendary but that's also because @thomasgutierrez (the immortalized 😁) sold loads of cards and diluted the market further. Soon that will be over, it's a sell wave.
The game system is crucial for the success in my eyes. If it's a well balanced, tactical and exciting game it could be a huge success for the Steem blockchain and also Steemit, more than the Kitties or Giftomon. I see a very challenging task to balance all the elements with specific abilities, special bonuses, upcoming items and so forth.
If there is no limited supply the prices would never rise above booster packs if you look at the distribution of each rarity class in my point of view. That would be bad for both, collectors and players because i think it's much more fun to have some very rare cards to collect and also to battle for. That's the stuff legends are made of! ⚔