An idea that I have had for a while now is an endless battle feature, this feature is unlocked once you can battle.
Basically you fight wave upon wave of ai endlessly with them progressively getting stronger. The first creatures that you fight are level 1 commons but they eventually get stronger and stronger until you are fighting lv 30 max boosted uniques and even apexs. Also you can stop battling after every battle with something like a “stop battling” button. Then once your ready to get back into the fray you can just continue where you left off but be careful, if you lose you will go back to the beginning all over again.
After every 5 battles you would get a reward starting off with a grey incubator and progressing into premium incubators, unique dna and (possibly) even apex dna of new apexs.
I hope you enjoy this idea as I think it would make the game more interesting.