To enhance the hunting experience, the ranger should be able to forge a illuvial tracking device which leads the ranger to the rarest illuvials that spawn on the current overworld run. It can show more specific hotspots on the map where the rarest illuvial spawns are, or it can direct the ranger to one of the 4 rarest illuvials on the map. So the ranger can go to all 4 illuvials and ensure that it catches the 4 rarest spawns. It could lead to the 4 rarest spawns in a random order, or from least rare to most rare, to make the hunt more difficult (would require finding the less rare 3 illuvials before finding the rarest one). It could also break, or be faulty, adding another element of grind to the tracker.
Could also complicate this idea by making it more like a pokedex from pokemon, where you can select an illuvial you want to hunt and it would lead you to it if it spawned on your run.