I'd love to see the Teen Titans crew show up. Trigon would be a good addition. Damian would be really fun to see. Donna Troy, she'd be great to see.
There's some good standalone stuff for Batman. The Killing Joke is great because you just need a general knowledge of Batman to read it, it's fantastically written and it's very short. For longer ones, Hush and the Long Halloween are some of my favorites.