I'm confused by the Shapechanger ability, which seems to be all over the place for different creatures. Here's the description of the ability from the Succubus stat block:
Shapechanger. The fiend can use its action to polymorph into a Small or Medium humanoid, or back into its true form. Without wings, the fiend loses its flying speed. Other than its size and speed, its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies
Normally I'd assume that means she can choose any form at the time she uses the ability- she could become the barkeeper in the tavern, or a small child, or one of the party members- whatever suits her schemes. But contrast that with the Doppelganger, which has slightly different text for the same ability:
Shapechanger. The doppelganger can use its action to polymorph into a Small or Medium humanoid it has seen, or back into its true form...
The fact that it's different got me wondering why they didn't use the same ability description for both abilities- does that mean the Succubus could shapechange into a humanoid she hasn't seen?
Then there's the Slaad, whose ability description in the stat block is identical to the Succubus, but has the following line in their flavor text:
Shapechangers. Some slaadi can transform into the humanoid creatures from which they were originally spawned. These slaadi return to the Material Plane to sow discord in the guise of their former selves.
So that implies they have one form they can change into, even though it's not stated in the stat block- but there are creatures who can only shapechange into a unique form, such as the Jackalwere:
Shapechanger. The jackalwere can use its action to polymorph into a specific Medium human or a jackal-humanoid hybrid, or back into its true form (that of a Small jackal)....
OK, so since Jackalweres can only transform into a specific humanoid, that implies that everything else can choose ANY humanoid form, right? But then there are Lycanthropes, the classic shapechangers, whose stat block reads:
Shapechanger. The wereXXX can use its action to polymorph into a XXX-humanoid hybrid or into a XXX, or back into its true form, which is humanoid...
I'd always assumed that Lycanthropes can only assume one specific form when they change, even though it's not stated in the text- for example, when Stuart the Wereboar Merchant takes his hybrid form, it's half Stuart/half boar, not any half-humanoid boar form. He couldn't become half boar/half Beauregard the Ranger to throw off suspicion, for example. That would make the ability more like that of the Jackalwere, although for some reason Wizards chose not to specify that.
This leaves me second-guessing my assumptions regarding Shapechange- does the ability a) allow the creature to take any other form unless specified otherwise, b) allow the creature to take a unique second form unless specified otherwise, or c) you're giving Wizards too much credit, and looking for consistency will drive you mad?
Since I'm writing an adventure for publication with a succubus villain, I can't fall back on DM fiat- if there's a consensus on this, or a clarification somewhere, my whole adventure will fall apart if I get it wrong.