Regarding quest rewards and late-game pve content in general, I would personally recommend having (quite a few) early-game quests that provide bonus packs/cash/etc, and having late-game content provide aesthetic boosts and other prestige effects that don't have any impact on gameplay. Examples are "foil" versions of normal cards, alternate card art (assuming we hit a point where we have enough art), alternate card backs, alternate playing field backgrounds, achievements listed on an account profile, etc. This gives the people who like playing a long pve game and/or who like having the prestige stuff something to do, while not requiring that people who are only interested in pvp grind a million years to have a strong card pool. Aesthetic/prestige stuff could be handled either via having it be direct rewards for certain quests/cheevos (i.e. complete this long late-game questline, get a pretty new playing field), as well as by having an aesthetic "shop" where you can purchase aesthetic boosts for large chunks of gold.