I've been thinking about this topic for a while - there are so many monsters that can polymorph/shapechange into other creatures, typically humanoids. Metallic dragons, slaad, couatl, oni, hags, etc. not to mention the classic doppelganger and succubus/incubus. 

In my mind, I think that the majority of these creatures should only be able to shapechange into a specific form, for example, Samantha the Gold Dragon can shapechange into a golden-haired/golden-bearded female dwarf (yes, I'm one of those "female dwarves have beards" people) - she can't mimic a specific individual, or turn into an elf, etc. Kermit the death slaad can change into a slighty froggy-looking human man, but can't turn into a woman. Argento the silver dragon is male... but his humanoid form is a female halfling. 

This will increase the "specialness" of creatures such as doppelgangers who are the classic "mimic and infiltrate" monster, who should be able to look like just about any humanoid. Obviously succubi/incubi would be in the same category, and probably hags as well... oni I am split on.. maybe some are better at it than others. 

There is, of course, a middle ground... perhaps a creature has a handful of forms they can take. I have read various fantasy or sci-fi stories in which shapechangers create a "portfolio" of set alternate forms. Maybe a rule of thumb would be that creatures with a "limited" shapechange have a number of forms equal to their charisma modifier?

Unless something is a shapeshifter "pro," I typically have them retain a bit of their default look. So a golden dragon is always going to have a dignified/regal look with golden hair. A slaad is always going to look a bit toady. Clues that reflect that they are a little easier to identify than a true shapeshifter.

I don't think there is anything RAW about limiting the forms a creature can take (except as dictated in the ability's description), but as a houserule I'm all for it, for certain creatures. Personally, I think metallic dragons should have free reign to choose how they appear (though they may by choice retain some aspect of themselves in their change, like a gold dragon having golden irises or a particularly metallic sheen to their (blonde) hair).  

Weaker and evil creatures might have greater limitations;  an incubus might only be able to change shape to remove their fiendish attributes like wings and tail, but keep the humanoid portions of their appearance.  Sladi might still look toady (bulgy eyes, awkwardly proportioned limbs, rough skin).  The more alien and malevolent towards humanoids, the less likely they can properly know how to mimic one with shapechange.

Changelings and Doppelgangers should not have any limits imposed other than those in the ability description.