#1: Here's a suggestion: Add boss eggs, they would be extremely strong mobs, their drops would give amazing rewards such as keys, spawners, money vouchers, mob coin vouchers and more. Boss eggs would be obtainable from every crate??? (Please don't force this to be only buyable in store)
#2: Another suggestion is artifacts: These "Artifacts" would be unavailable to use until you decrypt them at a npc somewhere in spawn, these "artifacts" would be gear that are insanely strong. Source of them: I don't know maybe some kind of event like envoy or bosses (if they get added)