Most of us already have problems with space for monsters especially the high level players. It makes us feel frustrated whenever we get a new monster but then we have to spend to make a room for that new monster. I have few suggestions for this.
If you already have made the monster to rank 5 it should also mean that you have already completely accomplished that monster. Instead of selling it or extracting it (collectors dont wanna do it), sp should make a new feature where in you can put that monster into some kind of book or in the monstagram as model and can only be summoned for breeding and war purpose only. This will lessen the monsters in the vault and habitat.
Vault keys should be categorized based on rarity. There are vault keys for c, uc, rare, legendary and mythics. The prices for this vault keys should also be based on its rarity. The higher the rarity the higher the price. I know SP thinks of money profit also, and should concentrate on mythics for profits. And also events for getting the keys should be introduced in order for you not to release more monster every now and then and it will be a new event as well. Introduction of new events will make the game less boring. It should not be concentrated on getting new monster as well as making space for it and events to make them ranked up and also for relics (some features are already existing).