Hrmm. Let's try and work this out logically then, starting with HQs. None of the new shit means everything Primaris is out, but in a Company-sized toolbox of marines you can probably find models that make good Lieutenants/Ancients. No Special Characters (that's gonna sting) limits the selection further though. Masters, Chaplains and Librarians in several varieties remain available at least.
Between Grim Resolve and the Dark Angel penchant for Plasma weaponry, a Lieutenant should probably be the first HQ. If we're aiming for some Ravenwing, putting in a Talonmaster would similarly be a good idea (if you've got a model that can serve).
Master might add some more mobile rerolls if we're footslogging, though I'd normally recommend just going with Azrael then...
What does the armoury look like? What kind of heavy/special weaponry do you have available?
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.