Any decent crossovers that manage to fuse the two universes? I'd really like to read one where an adult Harry deals with the madness that is Gotham.

 

Got your back man. Or should I say... Blackman. There's yet to be a fusion/crossover that surpasses its quality, in not just HP/Batman, but any story fusing/crossing over with any other story. In fact, I don't think even regular stories, be they original or derivative fanworks, have ever glimpsed the meteoric and incomparable caliber Blackman resides within. Not that I've looked, though. Looking is hard. Just like Blackman. All the time.

 

Read it. Guess I'll read it again. Hasn't updated in a while, though...*hint hint HINT*

 

The Wizard of Gotham - humorous HP/Batman crossover. Unfinished.
https://www.fanfiction.net/s/3509790/1/The-Wizard-of-Gotham

 

I liked Shadow over Hogwarts and it actually is finished although it left open the possibility of a sequel which hasn't been written.

https://www.fanfiction.net/s/6827897/1/Shadow-over-Hogwarts

 

To Master the Dark (Harry trained by Bane) - https://www.fanfiction.net/s/9520876/1/To-Master-The-Dark