You'll be surprised how many story-lines DC hasn't done Trade Paperbacks for. I'm going to list some I really want to see.
The Battle of Metropolis/Fall of Metropolis story-line that spanned 9 issues of Superman comics in 94.
The 3 part story that originally introduced Spoiler/Stephanie Brown from 1992 in Detective Comics. Throw some other stuff in to elaborate on it, like early Cluemaster stories to explain him, or later Stephanie centric stories from various Secret Files and Origins books, like the one where she sums up her history to her new born child.
Dead Reckoning, a Batman story that was going on in Detective Comics about the same time Hush was in the Batman title. One that has similarities to it but that some fans liked better.
The Noctruna Arc from the Pre-Crisis 80s Batman Comics, that would take more then one Volume. And then one including the Batman story of Detective Comics 566 and Batman 400, the last Pre-Crisis Batman story.
The New Teen Titans/Tales of the Teen Titans comics that deal with the aftermath of The Judas Contract, issues 45-55.
And I'd like to see a sort of prequel Trade for Bruce Wayne Murderer/Fugitive chronicling his relationships with Vesper Fairshild and Sasha.
I'd also like some Infinity Inc and Wonder Woman story-lines from the 80s.
And some Bronze Age Supergirl story-lines.
I could also complain about how many TPBs have left out things they should have included.
I wish the Who is Donna Troy TPB had also included the original Origin of Wonder Girl story from Teen Titans 22 in 1969 which was also written by Marv Wolfman.
Even the new two volume collection of War Games that includes many things the old trades didn't left out Batgirl 54 which is the last time we see Stephanie before the in-sighting incident. Also Catwoman 33 and 37 should have been included for the Prologue and Epilogue parts.