I am a bit surprised when non of the existing DLCs featured circles of slaughter with there number of new enemies featured in each DLC. Murderlins Magic Slaughter was one of my favorite challanging additions to BL2. It provided hours of progression and improvement from my friends and I. When the Handsome Jackpot dropped I was was really hoping to see a circle of Slaughter tucked away at the end. Instead I got the Double Downer shield form a long side quest.
DLC2 features even more new enemies with some crazy empowerment mechanics between enemie types. DLC3 has some new enemies but far fewer then the previous 2.
In reality I doubt GBX didn’t think of this. I feel like it is far more likely the choose not to or didn’t have enough time. I would love for them to go back and add a Jackpot and a Xylourgos Slaughters with unique environments befitting of the enemies. If vehical combat was at all balanced I would also recommend doing a distrustion derby type slaughter with DLC3.