I would like to have an addictive reason to keep leveling my perfect illuvials non-stop. Leveling an Illuvial gives it a 1% stat boost to Health, Omega Power, and Attack Power. It's a small upgrade per level, but for a perfectionist--it will drive us and keep us engaged long, long term--potentially for years to upgrade and increase our Illuvials power levels.
All of that said, I am not suggesting anything too extreme. I would recommend adding increasing of level only at extreme amounts of XP.
For example, Level 60 is 700,000 TOTAL XP. I would recommend the following (Total XP Level).
1.2 Million TOTAL XP for Level 61
2.2 Million TOTAL XP for Level 62
3.5 Million TOTAL XP for Level 63
5 Million TOTAL XP for Level 64
7 Million TOTAL XP for Level 65
10 Million TOTAL XP for Levle 66
15 Million TOTAL XP for Level 67
20 Million TOTAL XP for Level 68
30 Million TOTAL XP for Level 69
40 Million TOTAL XP for Level 70.
60 Million TOTAL XP for Level 71.
Can keep going at ever increasing amounts of XP...
To be honest, I highly doubt anyone will ever reach a Level 71 illuvial. Imagine the 10,000 of thousands of Stage 3 runs it would take to reach Level 70. But if you did, that Illuvials value would go up drastically if you wanted to sell it, because you burned literally million units of crypton to go in and farm that much XP.
It's good for the free market and its good for long term illuvial progression, good for getting players addicted to the game long term. Rewards extreme dedication to the Illuvium game and ecosystem.
I also don't think that this would drastically affect balance--due to the rarity of illuvials that could possibly be leveled above 61-63. It's so much playing to achieve.