Ability to buy each item in the store maybe at say 1 or 2 of each per month or make them more accessible.
You could for example even add to quests or titans etc something like these,
- make every quest that currently exist or some have a harder version which offers the player to pay say a few hundred gems for a guaranteed chance of a rare ascension item
- every quest and event offers the option to pay a few gems for a guaranteed chance of a 4* ascension item
In my opinion the game is fine for f2p player and even acceptable for c2p players but I feel those that spend or wonna spend more money to improve their gaming experience should be given an option to ascend their heros accordingly to reflect and justify there expenditure within the game.
I would go even go as far as putting something in place that provides say a free 4* ascension item for every 100 or 150 dollars spent. Even tbis would help justify spending somewhat.
Any of these would not only encourage spending but encourage the need to want to get more heros.
None of those will affect f2p or c2p players any more than the game already does.