If they did Blackstar as a 1-off with his own packaging that could work.
Use the MOTUC he-man buck, same size packaging, so that it was virtually a part of the line, but not directly and not by branding.
That way anyone who wanted could purchase him, but it wouldn't actually be part of the line, no problems with angry completionists.
Also it could sell right alongside MOTUC on Mattycollector.
If he did well they could do more - maybe 1 per quarter. (klone, etc.)
But, as others have pointed out Mattel doesn't have the rights and probably doesn't want them.