The game itself is pretty perfect.
What really needs to be improved is the order and convenience of everything. For example as a player who rents out a few cards, an easier interface for managing the cards I rent and rent would help a lot. Or notice of cancellation of rental etc.
Similarly when choosing the cards for battle, an option to create favorite cards that I use quite regularly (the chicken for example) would make life easier ...
On this subject they can take a good example from any other game, but the one I remember particularly well on this subject was X-COM. This game required management of several things simultaneously but it always felt so smooth and simple.