Not much is known about Leviathan arena right now, maybe the team has something in mind already how to attract whales, here is my idea.
While ascended should be absolutely balanced and turn/logic based, leviathan should be much more chaotic and action packed. If illuvials' power in ascended is limited, because of dimensional stability or whatever lore they want to put it behind, then in leviathan that stability is broken and the board is dynamic with different things happening.
I just had this idea. What if Illuvitars can be used as cards in leviathan only, to enhance 1 or more illuvials with additional spells or just use the Illuvitars as spells themselves, that can be used during the battle. Or it can be once you complete a certain line of Illuvitars, you unlock a certain spell related to that line, based on their power level. Different tiers of spells will be locked by the power level of ur Illuvitar, with strongest cards providing the strongest spells. A certain amount of spells or cards can be used per battle, to make the pace of battles faster paced with random elements and fast decision making needed.
That alone or combined with a dynamic board can create a much faster paced gameplay and in contrast to the one in ascended, so people have more options. Will also provide an additional demand for Illuvitars, including the most powerful ones and also incentivize whales and players to invest higher to compete in this mode.