Collection of them all?  not for me

Absolute of the best BATMAN: ELSEWORLDS?  Most definitely!

Let's compile a list ....

Howard Chaykin's  BATMAN: DARK ALLEGIANCES
Mike Mignola's  BATMAN/HOUDINI
I could go for Brereton's THRILLKILLER ... IIRC that was an adult Robin (as Batman), adult Batgirl set in the 60's story

An obvious one is the three Moench/Jones Batman/Vampire stories ... but that has already been a collected edition and I would get a dedicated Absolute, too.