@JeremyECrawford Druid turns into a bug, gets eaten by a humanoid bad guy, and reverts back to human form inside his throat. What happens? Explosive decapitation? Druid shunted into nearest space? Other things? No answer on SageAdvice
— Dacromir (@dacromir) April 3, 2018
Wild Shape can introduce wild situations. What happens when someone swallows a druid in a Tiny form? Is a druid fecund in beast form? The rules are intentionally silent on these corner cases, leaving adjudication to DMs. As always, I say go with what's best for your story. #DnD https://t.co/mjQsU51uWs
— Jeremy Crawford (@JeremyECrawford) April 3, 2018
The same issue arises with Enlarge / Reduce, due to a curious detail of the wording.
If you stick a small rock in someone’s mouth and try to Enlarge it, they’re fine because it specifically says that if there isn’t enough room, it attains the maximum possible size in the space available.
But if you take a big rock, Reduce it to the size of a small rock, and then stick that in someone’s mouth, upon ending concentration it will revert to being a large rock with… grisly results. (DM’s Note – this could easily help shift a PC toward evil alignment, since it’s basically like shoving a grenade in someone’s mouth.)
My rule of thumb is that magic is created to be safe for the caster primarily and, if it’s not an attack spell, safe for those around it. The displacement effect we see that is common for teleportation and etherealness spells/effects (the creature/object is displaced to a safe location and takes x force damage based on distance traveled) is how I would usually rule it. Enlarge or shrink objects then put them where they don’t fit? Displace them to a safe location and take a bit of force damage. Wildshape in to a location/person then change back? Displace and take a bit of force damage. Seems like the simplest and safest way to handle these cases.