The possibilities are endless! You could have the Doctor and Crowley teaming up to save the universe or Aziraphale getting caught up in a timey-wimey adventure. However, challenges could include keeping the characters' personalities consistent and not overcomplicating the plot with too many elements from both shows.
I think Loki and Crowley would be quite the unlikely allies. Loki is known for his trickery and self - serving nature in the Avengers universe, and Crowley from Good Omens is also a demon who loves to cause a bit of chaos. They could team up to play pranks on both the Avengers and the forces of Heaven and Hell from Good Omens. Maybe they'd create illusions to mislead everyone and have a good laugh while doing it.
I think Aziraphale and Lucifer would be fascinating. Aziraphale is an angel who loves earthly pleasures and has a soft spot for humanity. Lucifer, being the fallen angel, would view Aziraphale's views as naive at first. But as they talk, there could be some deep - seated realizations on both sides about the nature of good and evil.
Sam and Aziraphale could also be an interesting pairing. Sam is the more intellectual of the Winchester brothers, always researching and looking for answers. Aziraphale is an angel with a vast knowledge of the occult and religious lore. They could team up to research a mysterious supernatural phenomenon that neither of their respective universes has seen before. Sam could learn about some of the more angelic aspects of the supernatural from Aziraphale, and Aziraphale could be impressed by Sam's modern - day research methods.
Another great storyline could involve the prophecies in Good Omens intersecting with the apocalyptic events in Supernatural. The characters from both would then be scrambling to prevent or at least manage the impending doom. The Horsemen from Supernatural might interact with the Horsemen - like figures in Good Omens, leading to a battle of wills and powers that would keep readers on the edge of their seats.
One idea could be that Crowley from 'Good Omens' and Lucifer from 'Lucifer' meet in a neutral realm. They start off as rivals, each thinking they are the most devilish. But as they interact, they realize they have more in common than they thought. Maybe they team up to play a prank on some self - righteous angels, which goes awry in hilarious ways.
One interesting storyline could be the interaction between the angels and demons from Good Omens and Lucifer. Maybe the characters from Good Omens end up in Lucifer's LA and have to deal with his brand of chaos. It could lead to some really funny and thought - provoking situations as they compare their different views on good and evil.
Another idea is that the Avengers' time - traveling adventures somehow cross paths with Good Omens' apocalyptic timeline. The Avengers could accidentally end up in the middle of the Good Omens' end - of - the - world scenario. They might try to use their technology and powers to stop the apocalypse in the Good Omens universe, while Aziraphale and Crowley are both amused and a bit confused by these new, powerful visitors.
One way to create an engaging plot for this crossover fanfiction is to focus on the characters' reactions to the new world they find themselves in. Imagine Dean Winchester suddenly in the middle of London and being confronted by an angel and a demon who don't quite act like the ones he's used to from his own world. His confusion and initial mistrust could lead to some interesting dialogue and action scenes. Sam, on the other hand, might be more open - minded and start asking questions about the Good Omens universe. Meanwhile, Aziraphale and Crowley are trying to figure out who these strange American hunters are. This interplay between the characters from different universes can drive the plot forward.