Potential different option:

• Make Soulbound Reward cards "Ghost" cards for all accounts
• By adding them as "Ghost" cards, it allows all players, new and old, to use them at the max level in the league they play in (levels the playing field)
• Make these Soulbound Reward cards ineligible for any Daily Quest Focus bonuses unless the cards are owned by the player and maxed to the level of play
• Add an additional 5% RP boost per 'owned' Soulbound Reward card played in battle and won
• Soulbound Reward cards will not be "Ghost" cards for tournaments or brawls, only for Ranked play

The only unlocking allowed for Soulbound Reward cards should be for Land. They should not be allowed to be burned for DEC, or unlocked to be sold. The incentive to accumulate these cards for the Ranked play bonuses, advantage in Brawls & Tournaments, and ability to stake on Land is more than enough to keep players engaged in my mind.