As long as the hair pieces would fit any head, glued on are ok. Short to medium lenght hair probably would look better sculpted on, so maybe every/some faces would be available bald and with hair?
From there we also have lots of mask types: full head, full with visible hair, visible mouth (Captain America), visible hair and mouth (Captain Marvel), visible hair and face (Gambit, Angel), domino (maybe even different designs - classic, tied like Golden Age Robin, Nightwing, Kyle Rayner, Ms. Marvel's and Black Cat's wider masks with visible eyes), full mask similiar to Deadpool's - with the "drop" on the back of the head.
And for articulation, TB style is great to hear, though I prefer the Mattel WWE Elite style hip joints. I think they're the best looking, though their range of movement is limited.
Clif52 wrote:Maybe mix and match heads would work. Pick your body type and then choose the head of your choice.
I like that idea. You could have ready sets or a step by step shopping - first choose the body(bodies), then head(s), hair, accessories etc.