Make a special set of cards that can only be obtained by burning cards.
You have to burn 400 commons(any commons but maybe not soulbound) you get 1 bcx of a epic card.
You can burn 115 rares to get 1 bcx of a different epic card
These cards would have to be very good as it would require 18,400 common cards or 5,290 rares to make 1 max level epic.
Maybe even go a step further and require epics or legendaries to upgrade to the first mythic monster! This will help boost the card values and help them maintain a nice floor over an extended time. These cards would need to have some kind of limited print run or time limit to make them exceedingly rare.