I'm not holding my breath for any kind of DLC, for many reasons. However, in a theoretical scenario, a post-game Avernus DLC would make no sense logistically: for some characters Karlach died on that pier, and some characters killed her on Wyll's quest, and some characters attacked the grove so she never joined/left the party, etc etc etc. And we don't exactly have any other motivation for our character to go to Avernus post-game. There's no sense in releasing a DLC that hinges on one choice being required and arguably made "canon", locking everyone else out (and adding another severe enough reason post the big bad being defeated... too much needless work, nah)

So, if any (although I doubt anything of that scale), it would have to be before the ending, that's for sure.

[Linked Image from tarraxahum.neocities.org]