That's a good question. Hotel-girl is unlikely because Hunter and Drew probably don't know she exists, while Sterling has only casual interest towards her (so it wouldn't be much of a "bad" end). My guess is Kumiko, she's been built up for a while and hasn't had a full-on sex scene yet.
A long-shot that I'd like to see is the return of Keira (and possibly her mother). She looked great in her What If appearance, and I wonder if she's still enthralled to Hunter and Drew.
Alternatively I'd like to see more threesomes in the bad ends. In chapter 9 we saw Hunter/Catherine/Amber. I'd especially like to see Hunter/female/Drew spit-roast action.