While the Changeling's shapeshifting ability does provide the player with quite a variety of creatures (both pc and npc) to choose from, the rules do limit it to medium human-like (as humanoid is a type) creatures lacking tails (and arguably horns).
As an action, you can change your appearance and your voice. You
determine the specifics of the changes, including your coloration,
hair length, and sex. You can also adjust your height and weight, but
not so much that your size changes. You can make yourself appear as a
member of another race, though none of your game statistics change.
You can’t duplicate the appearance of a creature you’ve never seen,
and you must adopt a form that has the same basic arrangement of limbs
that you have. Your clothing and equipment aren’t changed by this
trait.
However, I am left to wonder if this means that the changeling is barred from accessing the image of nearly half the PC list, or if the wording of "a form that" suggests there are ways for a changeling to overcome their limits, relying on RAW?
For instance:
- Could a changeling personalize the visage they use beyond the race's traits, such as a Drow with black hair, a shifter's shifting features, or a tiefling that lost its tail?
- Could a potion of giant size, or the Enlarge-Reduce spell allow a changeling to temporarily dip in the visages of other races?
- Would consuming "Blood of Lycanthrope" give them access to the tail due to the hybrid form?
- Are there any feats yet in 5e that can allow the changeling to upgrade his shifting from alter-self like to disguise self like?
As a side question, I noticed that changelings don't appear to have darkvision like their doppelganger cousins(?) do. Did I miss this point or would this also apply a disadvantage to the kinds of masks a changeling can effectively use?
Best Answer
The changeling is more flexible then you are assuming
There isn't anything in the Changeling race that precludes adding tails or horns or other visual aspects.
The only requirements are what you've quoted, and nothing about horns or tails would qualify them as limbs.
Specifics what is allowed at a table will be between player and DM, but as long as you aren't adding anything functional and keeping the same creature size and limb arrangement, you're good.
To answer your section on enlarge/reduce, this would open up more possibilities when you hit the size problem. Moving to a small or large sized creature via that spell would allow you additional humanoids to shapeshift into (otherwise, you're just a really weird sized version of those races.)