In all existing game modes I can see one and only AI that matches gems like a newbie. It goes for skulls, misses some 4-gem matches (sometimes even evident ones), converts gems not effectively. People already got used to it and plan defense teams respectively.
I suggest to add another game mode (pitily that Challenges were reworked already) with much more clever and cunning AI. Which won’t miss some 4- or 5-gems match, even one not plainly visible. And converting gems will take into account 4 or more gems matches to have control over board. I think it will be very challenging for mid- and end-gamers.