So of the Mk7 helmet style HPP, we have these, bold if they parts exist to make them, italics for Shotgun derivatives-
Mark 8 (Mk7, Centurion, Striker parts)
Mark 10 (Mk7, Hammerhead, Heartbreaker, Snapper parts)
Mark 13 (Mk7, Centurion, Shotgun, WM2 parts)
Casanova (Mk7, Centurion, Heartbreaker, WM2 parts)
Tiger (Mk7, Hammerhead, Mk7, Striker parts)
Midas (Mk7) (already made)
Fiddler (Mk7, Centurion, Heartbreaker, Starboost, Striker parts)
Romeo (Nightclub, Centurion, Heartbreaker, Mk7 parts)
Southpaw (Mk7, Centurion, Snapper, Shotgun, Heartbreaker, Striker parts)
I'd say not Mark 8 as 9 is already in the works with a close release date and they're both red/gold bashes. 13 and Southpaw needs Shotgun parts but dunno if Shotgun is far enough along but its a potential. 10 and Tiger need Hammerhead, so they're out. Romeo needs Nightclub, out.
So I'd say potentials are Casanova or Fiddler, Mark 13 or Southpaw if they're ambitious with Shotgun parts.