barren ember
Rather than finding them directly roaming on the map, enlarge the maps and add procedurally generated enemy dungeon
You land in the starting area and enemy dungeons are scattered on the map, and the illuvials are inside these dungeons with the enemies of the heroine and will bring Lore with Lore pieces to collect. And you will have to eliminate the enemies to succeed in saving the illuvials kept prisoner, the enemies will also have firearms and melee weapons, and you must not die under penalty of returning to Sanctum Mesa.
It will bring more fun by adding FPS battle phases, we could also imagine later choosing one of our own illuvials that we have captured to offer us one or more special skills to fight and help us in combat in the overworld.
The enemies will also be able to drop resources
We could also imagine the addition of bosses or mini bosses with better loots. Or more rare illuvials in donjon.
Finally why not give the illuvials the ability to fight against us in the overworld so that the capture is more or less difficult depending on the rarity and stage of the illuvial encountered, which will also force us to be much more careful.
All stages for free once a day :
Remove T0 and once a day to play free T1 T2 and T3 stage, 3 games so at least 3 hours of play with the bastions which seems suitable for a casual player. But by dividing the chances of rare illuvials by 10 for example. This would give an interest and a way for free to play players to extract value and really have an interest in doing their daily games.
Add a daily connection bonus.
Each player, whether invested or not, should be entitled to these 3 free games per zone and per day, in order to hook the player and not necessarily put their hand in their wallet. Only those who wish to pay will be able to do so to play additional games with a chance of rarity 10x higher than the free games. But still limit them to a certain number per day
add skins/goodies that will be put in the ILV pool