Actually, think there is a better way for this.... " Illivial Encounters”, at specific locations on the map there will be an “Illuvial sphere” where the ranger zone into a new area ...."
If these Illuvial Encounters was made like a mini Dungeon, where you fight your way to the final room... which could be made as the Battle board, and as you enter, the sphere close around the area. and the battle begin against a team of Illuvials, and if you defeat them you get to attempt to capture one of them.
As you enter the Mini dungeon you get to choose the difficulty, between 0-5. this also determine what level of Illuvials will spawn as random in the end. (level 0 will be free and only spawn T0 Illuvials, Level 1 will cost 5 Fuel, and spawn T1 Illuvials, with random stages... Level 2 cost 10 Fuel etc.) . the level 5 Mini dungeons should be hard to complete and will require a strong team, with decent ranger equipment/level, but can also reward the chance to capture a T5 Illuvial. (might be able to make these encounters with multiple of your Illuvials fighting a long your side... if you have the skill level to support them).
These mini dungeons would be "single player" where both the OW and "Rift dungeons" would be multiplayer.... but I think it could actually work fine, as it also provides more diverse content... even if a group of 4 people enter the OW together they can fight together in the OW and Rifts, and then go to the Illuvial encounters and enter them by themself.. but they get to "chat/talk" about how the encounter goes for them... and they can choose different levels, so they also do not have to be at the same level when they enter the OW. they should also be fairly fast to complete so they will not have to wait that long for the others run.