I had this idea and wanted to know what you all think about it.
You could swap the levels between two identical Illuvials. For example, your main Atlas is level 45 with decent stats, and one day you capture an Atlas with perfect stats. It feels bad that all the levels you've put into the first one are practically useless, and now you have to level up the new one from scratch.
The cost for this swap should be fairly high, making it much more cost-effective to level up manually. This way, if you don’t have a lot of money but have the time, you’d likely avoid the swap.
This mechanic would give whales the opportunity to swipe their credit card to speed up progression. However, since you can't directly "buy" experience, you'd have to purchase a high-level Illuvial from another player and then pay the fuel cost for the level transfer.
It would increase the value of low-stat, high-level Illuvials and reduce the frustration of leveling mediocre ones, where you "lose" all the progress once you upgrade the illuvial to better stats. While it might decrease the value of pre-existing high-level Illuvials, it would overall improve the game for important player demographics.
I believe this would increase both Marketplace purchases and Fuel consumption, while also boosting player satisfaction within a key demographic.