As long as this game exists, there are “random” rules in card drops. It was totally fine while we had tenths of cards, it became seriously annoying when it became hundreds, and now it’s pure madness, unfortunately. 
- Chest drops. As only rarity regulated, I may never get some cards, and got other in too big amounts. Most visible with mythics, where I am missing dozens of them, but now and again getting duplicates of one I have long ago. I have 3 Amaroks and 3 Jotnars, for example. But when I checked unowned cards, I noticed that I am missing lower rarity cards too! Knightsong? Rattigar Gladiator? Fell Hydra? Why I never encountered these cards?
- Dragon egg drops. You know, Dragonite isn’t exactly freely available in the game, and as there is no rules against random, it seems, chance to fill all 6 dragons isn’t too good. I bought 11 eggs before gave up and bought Topasart in store. And I have more, and more expensive eggs ahead.
- Gnome rewards drops. I have tons of duplicated gnomes and tarot cards, which I don’t need long ago, because I have 4 copies of max rarity. And still I get them again and again. It’s a very annoying, especially for Epic rewards.
Maybe I missed something else, but you got the picture, I think.
So what I would want to see?
- For chests and chest-like drops (like eggs):
- Rarity of the card selected first.
- Users always got the missing cards in a given rarity, if such cards are in the pool.
- If no missing cards left in a given rarity, users always got the cards which they aren’t maxed yet (and not have amount enough to max them, probably? Not sure about this one)
- If users have all their cards in a given rarity, and all of them are maxed, well, that’s it - only pure random remains.
- For a Gnome rewards:
- Same as Sentinels for the Clan chests: if I have the 4 cards of max rarity, it removed from the drop pool.
I think, it would greatly benefit the players, and hardly will harm the Infinity profits - there are too many suggested pays in the game anyway.