• the joker fell in love with dinah lance (black canary)
  
• batman got visited by an alien batman from the planet zur-en-arrh
  
• batman got erased from existence except he didn’t he actually got sent back to the stone age before slipping into other eras and living as a pirate, a cowboy and a witch-hunter
  

Don’t forget my personal favorite

Batman was Jim Gordon for a bit and he hunted a flower delivery man who looked like the slender man and sold seeds that gave you superpowers

A few more:

• Jason Todd turned into a tentacle monster and started eating a person
• Batman knew Sid Vicious when he was young and stayed with him for a few weeks. After Bruce left, Sid “turned his predatory hunger” to Nancy, which eventually led to her death, and thats why Batman hates rock music.
  
• (Also from the same comic as the previous one: Two-Face listens to ABBA)
  
• Batman turned into a toddler and Robin had to take care of him
  

• Also from the above comic: all the inmates of Arkham Asylum enjoy rock and roll and Batman thinks this proves it is Evil. He took a young Dick Grayson to Arkham and threw him in with the inmates to try and scare him away from listening to it
• There is a crimefighting cow who is part of the Bat-family called Bat Cow
  
• The leader of the Secret Society of Supervillains was Alfred Pennyworth from Earth 3 who was literally a supervillain and he nearly destroyed the entire earth and killed the Justice League
  
• Batman once fought an evil ancestor who claimed to be his father, Satan and a billionaire movie producer and who tried to plant proof that Thomas and Martha Wayne were abusive nymphomaniac coke fiends and battled Batman with a Club of Villains made up of weird stereotypes from all over the world
  
• One of these was a man called El Sombrero who made his living designing death traps for less imaginative supervillains and another was a killer mime from France