The game currently has too many game modes for how small its community is. Furthermore, almost no one is going to want to play the current leviathan mode. Who wants to play a mode where you are going to have a huge disadvantage, and you are going to lose just because the other person paid more? Only people who have decks with near-perfect stats. You could say that people will play it if rewards are given, but if you have to pay someone to play something, that means that game mode is not right.
What I propose is very simple, that there is only one arena mode where the statistics of the illuvials matter, but little. These percentages would have to be tested but, for example, imagine that an Illuvial that has 5 life points obtained an extra 5% of life compared to an Illuvial with 0 points. The idea is that the player who does not have illuvials with the best statistics can continue winning, and can tolerate that disadvantage being small, and that the player who spends the money to buy those rarer illuvials, or invests time in capturing them and By evolving them they obtain an advantage, a reward, giving meaning to those rarer illuvials. The ideal is that in low elos that small advantage is barely noticeable and the important thing is the player's skill due to the low level of the players, and that it is in high elos where he faces good players where that advantage is really noticeable, although it should be able to continue winning with worse illuvials. Holo and Dark Holo illuvials can be made more powerful to give them a higher value and the difference is not just aesthetic, so a Holo illuvial could have 1% more in all its stats, and a dark Holo a 2%.