sinful depot
In the overworld, I think there should be something like gym battles from pokemon implemented where there is one difficult boss for each region and the bosses primarily have high-level illuvials of the type from that region. It could be a multi-round system like survival where you play against this boss NPC and you have to clear waves up to level 30 or something. The rewards for beating these bosses could be a unique cosmetic item, materials for crafting and/or an illuvial that the boss used during the battle. If a cosmetic item is rewarded, the skin may always be the same, but the item that the skin applies to could be random. This would encourage replayability to get all the items with a particular skin for the full set. Maybe each time you beat the boss, you have to clear further and further waves to get the next cosmetic item. It could also just be an upgrade cosmetic system where each time you beat the boss, you get a cooler-looking variant of the same skin.
Additionally, random NPC rangers within the world would also make the game feel a bit more alive since there isn't a multiplayer component in the overworld. This system could be similar to survival where each ranger has a set deck and you have to complete a certain number of waves to beat them. The rewards could be materials or even an increase to energy so that a player could stay within the world longer.
Finally, an elite 4 ish + champion like series of bosses could be implemented by adding an additional region to the game later down the line where the journey to get to this hypothetical elite 4 would be difficult to traverse such that a highly upgraded jet pack, boots, etc. would be required and illuvials within that region would be high level and may force you to battle them.
Like every other encounter, bosses and NPC rangers should use energy so that there is some risk of losing.