Which ABBA songs are they using for the sequel, they pretty much used every single hit in the first one - what's left?
You're absolutely right about that. I just looked at a list of all of their hits (global), and MAMMA MIA! actually used all of their big hits. Even "The Name of the Game", though it was deleted from the film adaptation was indeed filmed so its exists and can be seen. Heck, even though "Fernando" was used in any context it is heard: Donna is humming it and saying a few main lyrics when she stumbles across her 3 exes as she's making repairs before leading into "Mamma Mia!". They can now use "Thank You for the Music" which they didn't use for the film adaptation but is used in the stage version.
"Summer Night City", though it was used during early previews in London, is the only not-used hit song that most likely will be included along with the early European hit "Ring Ring." and the later "Head Over Heels." Every other song they had that charted is unfamiliar to the masses except the true diehard ABBA fans. Heck, I'm an ABBA fan and I don't even know them.