So, my honour mode run was a weird one.
I chose to ignore Gortash and not go to his inauguration, because reasons. Instead, I went rather straight for the Iron Throne and Steel Watch. By doing this I screwed up the Duke Ravenguard questline as he wasn't at the Iron Throne yet. Oh well... Never cared much about his or Wyll's questline anyways. Only, now the "Blade of Avernus" never came to be.
After defeating the brain, I had the following scenario:
My Tav was in a romance/relationship with Astarion and also at high enough relationship with Karlach to choose the "go to Avernus together" route. Little did I know that picking Avernus would for some reason completely negate the standing relationship with Astarion. The whole epilogue he treated Tav like there's never been any relationship, which I thought was really sad. Especially for an immortal being and the deep trust that had been built. A couple of months/years waiting time apart shouldn't be that hard to imagine.
Long story short: I would have loved to get a conversation option, that acknowledges their relationship and maybe even voices hope or longing to soon be together again, catch up and make up for lost time.