Anything given by Torgue, especially if they reference being Badass and/or EXPLOSIONS!
And ones with Dwarves from Tiny Tina’s Assault on Dragon’s Keep, because… reasons.
And a few more maps and missions for Athenas, the most underutilized planet in the galaxy.
Maybe something to illustrate the connection between Athenas and Sirens.
I would also like to see the whole premise of DLC3 redone using either Marcus or Clay as the narrator and main quest driver. Having no NPC’s from the main game in the DLC was just a horrendous idea from a franchise narrative standpoint, especially since they used a Jakobs planet. They could easily have done something with Clay chasing down old records for Jakobs and asking the VH to investigate the planet and helping to drive the mission flow. Or at worst have Marcus, the premier storyteller of the franchise, narrating the new story so that there is at least some tie-in to the existing BL3 universe beyond reused bandit frames and Eden-6 fauna. As it stands now, there is no reason for me to care at all about anyone or anything on the stupid planet (see, I don’t even remember what the planet name is. That is how non-memorable the DLC is for me!).
Maybe a few dozen actual missions for the 4th DLC.
And please bring back themed Headhunter Packs, but with a few more missions and things to do per pack and have some be re-playable beyond just a boss raid (but include a boss raid as well, of course).
EDIT - duh, and a new Bunkers and Badasses DLC!