There is. You win and you level up. Each new level you get some gems. Get to a new league and you get a lot of gems. You also get tickets which when you collect enough you get an Event card. Then, at the end of a season, you get free packs based on the rank you earned in a Season. I think there are plenty of rewards already to be had.
If you are bored playing the game, suggest some changes that will make it more interesting for you, but I think the rewards avenue is a non-starter.
Yeah, 4 cards per week (or 1 superare per week if you already have it, 1000 sparks). +1 pack at the end of the all challenges.
700 or more gems per week with ads, in the worst scenario a pack delivers 150 sparks. Almost 1100 spark per week or more. plus the gems of the league. Plus quests.
all this if you do not want to spend money in the game, I think it's fine, as long as the ads work, it's what matters most.