In favour of daily quests.
I think a battle should be begun immediately when you begin the quest rather than you accepting the quest and versing normal AI.
Not sure but this would probably be easier to code and easier to reward.
If the AI were given a unique creature or ability it might make the quests both interesting and a challenge.
For example if the AI had an immaterial pet with the ability "steal" from the start.
Pets art could all be the same with some colour changes, like the nymphs, to prevent too much trouble with images.
Other examples:
A pet with 100 HP and overdrive from start.
Permanent sundial in play - AI has fahrenheit and/or poison creatures
A pet with ability "lay fate egg" (could be very interesting - or just spawn random dragon)
An immaterial pet with ability " Parallel universe"
A row of 6 mindgates with 0 cost in play from the start
Pet with ability "Firebolt/icebolt/drainlife"
By unique I mean the player could not get these cards.
A final idea would be an "endless mode"
You would fight each FG in sequence (not random) retaining your HP from each round.
Your deck could not be changed.
In the event you defeat all the FGs they would gain additional HP and mark values.
A reward for this might be getting a single card from each FG you defeat.
I cant code. I'm sorry if suggestions are bad because of coding issues.