Not sure if the developers will see this but been a fan of these games for years.
Having said that, I'd love to see a survival mode for this game. By the time you get the Lord of Hell, you've completed everything so there's really no incentive to use him. Having a survival mode that runs in waves would solve this though, maybe getting a reward for completing 100 waves would be nice, and then having a max wave reached score. It would greatly increase this particular games longevity imo.
You should be able to sell cards in the currency they are brought in imo. I think that would make the game more well rounded with currencies - getting DPs is ridiculous at the moment.
Discovered something rather cool - if you complete a node, all the unopened treasure chests in that node are free. Not sure who actually knows about this, sorry if people were already aware, but I saw high rated complaints about it.