I am going to tag you @BigBadDesignermon because I do so sparingly and I believe this is something that should be considered.
Players want long evolution lines, things worth grinding for, and endgame.
Consider my question in the ask-a-dev thread where I asked what could be done about the discrepancy between players that spend smaller amounts of money over time and players that spend a lot at once. You said there was no good solution for this, but I believe there is.
Consider the Omegamon event. What if you could make Omegamon SP+ through the fusion of 12 level 60 Omegamon SP? We all know it was a huge pain in the rear to make him (and WarGrey and MetalGarurumon). It took an investment in batteries and the mons were average. I believe getting a SP+, even if it was a huge grind, would be worth it to many players.
Consider Beelzebumon SP+ Blast Mode. What if there was an event whee you could make him using a buttload of fragments and Beelzebumon God+ (or Baalmon like in Xross Wars).
These are ways to keep players that spend consistently happy, players that are in it for the grind, and players that love a good endgame.
I will give you another good one. Jupitermon Wrath Mode LR. What if F2P could make him. Yes, I know, make an LR? Sounds outlandish. However, what if he had evo requirements from Aegiomon's quest that make Imperialdramon PM look tame? For example, 6 Bursted Jupitermon God+ of each race? That would reward hard working players and is also steep enough to keep him out of the hands of new and casual players.
And if only 1 Legendary Rare in the game could be made, it should be Wrath Mode given the plot of Aegiomon's quest.
Some things to consider, BigBad.