Instead of selling new cards for the next major set, focus on reducing the number of extra cards out there and make money at the same time.
The idea is a set of new cards that are just new skins for the current cards. Splinterlands could sell a new token instead of packs. The new token would be used when burning old cards to generate new cards. The new cards would be some kind of morphed version of an old card with simmilar stats but maybe instead of 1 ability it gets another.
A quick example would be something like this,
You burn 5 bcx of exploding rats with equal CP of the new token(100 in this case) and you get 1 bcx of plague rats. This card will get poision instead of blast. Just as an example. Then maybe at level 5 it gets sneak instead of redemption and the art changes.
This will encourage burning of old cards for the new ones. Could change the set able to be burned every few months so the team has time to balance and test each set. Burning alpha/beta cards would help too as the new cards would be part of the modern set instead of just wild.
Cost of the new token should be balanced with the number of cards needed to burn to get a new card too. Something could be done with GF also.
This will make packs scarce and reduce the number of extra cards in the market while the team can still earn off of it.