Most definitely The Barry Sax and Lady Mal (UNLESS) and Duke Slick
Baron was not only a cool fluffy addition to the group, not only a highly mobile HQ, but also made hellions a damn sight more useful and opened up a bunch of new strategic angles with his stealth field and weird haruspicy.
Lady Mal was just always cool, fluffy, and has more than proven herself able to stand in the maelstrom of Commorrite politics. Seems like a real sensible choice.
HOWEVER, if she's like, just Yvraine now, I'd definitely say Kruellagh. The name is (was?) great, but I'd also be ok with them changing it to something a tad less on-the-nose. Her weapon concept was always really goddamn cool to me, and I feel like she could make a really interesting choice, especially leading a group of incubi or maybe witches.
The Thin, Poisonous Duke would be super great, if only to give us a more tangible option to go full-on sky-fall-deep-strike nonsense, and that's just too much fun. Also any poison buffs are of course welcome. It'd be fun to see him on some kind of venom-chariot or something. I feel like that would fit.
I really dig the idea of the Decapitator, and I think he could be an interesting character and a super cool model, but not necessarily an HQ. He kinda seems like more of a loner and a free agent, especially now that he finished his skull portal thing. I super dig his whole 'choose a head to take' biz - i would absolutely like to see that mechanic somewhere for sure. He could be like some sort of Drukhari death leaper.