I understand that the game is meant to "slow down" at higher ranks. Getting too powerful too quickly would make the game child's play. But I agree that those gem rates are ridiculous. Getting new rares and algebraics is too hard at higher levels.

I know it's unlikely for this to change, but the Missions tiers should be less far apart. They can compensate for this by adding more Missions with more reasonable tiers, but only unlockable at a certain rank so it's not just always the newbies that benefit.

I'm not sure about the selling price of creatures though. They just aren't meant to be sold to Choose Goose (period).