So you are writing a portal fantasy. You send your hero through the portal. He adventures. He comes back to the portal and passes through to find himself once more in the fields we know.
So what happens next?
Well, the best thing to do is to emulate The Phantom Tollbooth. Show us that your hero learned something in the other world and will now face his old setting with this lesson.
Milo had mastered perhaps the lesson easiest to show: he realized that his own world was well worthy living in. Other lessons, turning on the character's character, may need more artful handling.
In particular, because it's a balancing act.
Assuming, of course, you don't want to just create a fantasy world with the aim of teaching the character a lesson, and people it with fantastic beings whose purpose it is. Alice's Adventures In Wonderland and Through The Looking Glass, And What Alice Found There survive chiefly because Carroll made it a girl's pleasant interlude in a world in nonsense, the lesson merely being a tale that she can tell to amuse other children, a lesson as elegantly simple as the Tollbooth's.
True, the Disney movie reworked it to shift the lesson: the perils of a world where "everything would be nonsense." That also worked. (Notice that in the book, the king pardons people as quickly as the queen orders their execution, just in a lower key; this doesn't happen in the movie.)
The downside is that the fantasy world meant to teach a lesson loses reality. If you can shake a queen into a kitten, well, why not? Things do not mean things in their own terms in their own world.
The progress of fantasy through that era, to The Lord of The Rings and beyond, is to make the fantasy world real, and as a consequence the things that happen to it and its inhabitants are real and therefore important. It is not for nothing that George MacDonald had Mr. Raven, in Lilith, caution the hero, "As for moral laws, they must everywhere be fundamentally the same."
A world in which shaking a queen into a kitten is as evil as any other form of involuntary transformation magic is more serious in itself than Wonderland.
Much as I love the movie Labyrinth, the labyrinth is filled with call-backs to her room at the beginning of the film. Were the goblins drawing on her creativity to take on form? (Would explain why they want Toby.) Were they a pop-culture influence in her world, so that the goblins were naturally drawn to a person who liked their appearance? Or, possibly, since the labyrinth follows her maturing from egotism, are the goblins manifestations of her thoughts?
Well, while there are ill-conditioned souls who insist on any fantastic element being a metaphor, it's clear enough that the goblins are real, but it's teetering near the edge.
One notes that the force that propelled her into the labyrinth -- the portal -- is clearly a character in the story. The thing is a set-up to trap her in her own character flaws, her egoistical sacrifice of aesthetics to her selfishness and spite, and so to succeed, she has to rise above them.
This is a good rule. The reason the character is in the other world should not be a mere plot device. Another character choosing this one makes it more plausible that the character is well suited for the problem that the other world faces, and benefits from dealing with it.
Usually a more beneficent force is involved, choosing the character and putting him somewhere for the good of all non-malignant characters involved. This character, not being the villain of the piece like the Goblin King, can be more mysterious than the king is. Indeed, this character generally benefits from being mysterious. Sagacity, judgment, and knowledge can be implied better than they are mostly off-stage.[see here]
And all three abilities are needed to coordinate the character's inner story with the world's problem. Some lessons are easier than others -- the grass is always greener on the other side of the fence, or be careful what you wish for -- but all of them require artistry.
A thought provoking piece. "Ill-conditioned souls," indeed!