To deny leaving matches -  after each match a crate drops, the crate can contain something as useless as 100 crowns, could drop various low amount or crystals with 5% chance, but has like 1% chance for dropping a chest key, 2% for dropping a "Rare crate", 0.5% chance for dropping an "Epic crate"
New use for chest keys - use 3 chest keys to open "Rare crate" for a GUARANTEED Rare item, like 5 to open "Epic crate" for a GUARANTEED Epic item.
That is about 500 matches to get enough keys for Epic chest open, but 200 matches for Epic chest drop - motivates ppl to buy more keys - more monies, but at the same time more Epics for everyone.
300 matches for Rare chest keys, but 100 matches for rare chest - motivates ppl to buy more keys - more monies, but more Rares for everyone.
You could use those monies to create more content and motivate ppl to buy yet more keys :p
Sounds like a plan imo, u du dis pls. Not sure about the numbers tho... adjust it as you want...