faction/subfaction specific items and more weapons and armours is my vote heros/elites with their RP cost/CD have soured in my eyes, housing options seem both pointless and a waste, squad/SF options are out of my area of concern, pets are well.....pets, so who cares
I wouldnt have any problem with dropping a dime for cosmetic customization, seeing how its indifferent on tabletop either.
Cosmetics for the individual, including chapter/clan/legion specific items etc should be the priority out of that list imo. These are the sorts of things that are going to define Founders to the rest of the community long after release.
Slaaneshy as in Sonic Blaster, Blastmaster and Doom Siren weaponry or Slaaneshy as in giving CSMs alternate voices, boobies and lashing tongues cosmetics ? Cause either way ... I'm in.