Okay so I recently learnt that John Constantine exists. And I love British rat men who don't have their shit together. But since I also know very little about the DCEU and didn't finish the Hellblazer comics yet, I just decided to ship Constantine and Batman. As one does. Anyway, here's an AU idea saved for the future:
Fake dating AU, where Bruce needs a fake date for some reason or another. Something something publicity, he can't use Clark or Diana or anyone else for an unspecified reason. John doesn't know Batman's true identity. Bruce has never heard of John, so he has no clue what he's getting himself into. They meet at a high end bar. Bruce looking for a fake partner, John riding the synchronicity wave and being in the right place at the right time (perhaps he's investigating something). John hits on him and Bruce realises, with a bit of cleaning up, John qualifies for high society and makes his proposal. John agrees because why not, could be fun, nice change of pace.
The plan works out smoothly, John can behave himself if he wants to, Bruce is nice company, so there's no loss there either. Except some time after, someone in the Batfam (I don't know who, I wish I knew them better) comes up to Bruce and goes "hey dad why the hell are you dating John fucking Constantine" and now Bruce hears about his reputation and oh god what has he done. Hijinks ensue. And yes, it would be a Batfam fic as well. Constantine having to deal with Bruce's kids and vice versa. Nobody really knowing how to handle John and John himself signed up for a hot fake boyfriend, maybe a bit more, but not that. And why the hell is Batman stalking him on his missions?