Thanos could be an interesting antagonist in a crossover. Imagine Dr Fate having to use his magic to try and stop Thanos from getting all the Infinity Stones. Thanos' brute force and determination would be a great contrast to Dr Fate's magical finesse.
One idea could be a crossover with the Justice League. Dr Fate could use his magic to help the League against a powerful magical threat that they can't handle alone. Maybe a dark sorcerer from another dimension is trying to invade Earth, and Dr Fate's knowledge of ancient magic is crucial in devising a counter - attack.
One idea could be to explore how Hercules' incredible strength affects the Fate universe. Maybe he is summoned as a Servant in a Grail War and his brute force challenges the traditional battle strategies of other Servants. His Twelve Labors could also be incorporated in some way, like each Labor being a trial he has to face within the context of the Fate story.
A third idea is a crossover where the characters from both universes have to team up to face a common enemy. Maybe an ancient evil that threatens both the world of Fate and the DxD world. They would need to combine their powers and strategies, like the devils using their demonic powers along with the Servants' unique abilities.
The fanfiction could also delve into her origin story. Maybe she was hidden away from the world for her own safety because of the danger her powers pose. But as she grows older, she decides to break free from that protection and join the fight. This could create a lot of drama and action in the fanfic.
I like 'Dr Fate's Apprentice in Young Justice'. In this fanfic, a new character becomes Dr Fate's apprentice. It's interesting to see how they learn from Dr Fate and how the dynamic between them affects the Young Justice team. It also has some unique interpretations of Dr Fate's magic and how it can be used in different situations.
It would be really wild. Harry Potter in the role of Dr Fate would bring a whole new dynamic. Harry's magical skills from the wizarding world combined with Dr Fate's powers and responsibilities. He might use his wand in new ways while wearing Dr Fate's helmet. Maybe he'd use his knowledge of Hogwarts magic to fight enemies in a different, more cosmic way like Dr Fate does.
Another theme is redemption. Sometimes, the fanfictions will explore the idea of Dr. Doom trying to redeem himself in the new universe he has crossed into. Maybe he sees something in the new world that makes him question his old ways.
One of the best Dr Fate stories is 'The Tower of Fate' saga. It delved deep into the mystical realm associated with Dr Fate and his connection to the Tower. It showed his battles against powerful magical foes and his growth as a character.