I recently read a book that got me thinking about portal fantasy again. (For my earlier thoughts, see On Portal Fantasy.)
It was itself a recent portal fantasy, and the way in which it got me thinking about the genre was not a good way. The protagonist goes to another world, where the inhabitants are suffering under a dictatorship. It's ripe for change, but the people themselves aren't able to bring that change; that needs the special person from our world. She triggers the change by introducing a different element to their act of religious worship, which, instead of finding deeply offensive, they welcome.
After some vicissitudes for her, the domino effect from her (relatively minor) action brings down the dictator, whereupon she turns around and goes back home, leaving the locals to sort it out from there. (Of course, nothing in real-life history suggests that there will be any kind of problem at all when a place with a diverse population that has been under a dictator for many years is finally cut loose; coughYugoslavia, coughIraq, cough, sorry, something in my throat.)
If you are thinking, "That sounds like the White Saviour trope," that's what I thought too--except that in the book, the protagonist is not white. It is still pretty colonialist in its effect, though.
I posted on the Codex writers' forum, of which I'm a member, about this, and the resulting discussion has helped me work through some further thoughts on portal fantasy. Discussions on that forum are confidential by default, and I don't want to take anyone else's ideas and claim them as my own, but I will talk about what I came up with myself, acknowledging that it was the context of being able to talk them out with other people that helped me develop them. I'll also briefly mention immigration, which was discussed by other people in the thread, though I don't have a lot to say about it myself.
So, a theory: portal fantasy is one part of a wider group of genres that also contains (some) utopian fiction, Lost Worlds, fantastic voyages, and (some) planetary romance. Possibly (some) alternate-world-hopping fiction, (some) multiverse stories, and (some) time travel also.
It's sometimes said that the only two stories are "someone comes to town" or "someone leaves town". This genre is about "someone really leaves town". It's a visit to a place that is strongly Other, and what happens there says a lot about how we feel about the Other.
Early on, when most people didn't travel much; travel was difficult and dangerous; and people not very far away were extremely different, the Fantastic Voyage predominated. Think Jason and the Argonauts, the Odyssey, early medieval examples like Brendan the Navigator, the later medieval Travels of Sir John Mandeville, and, as late as the 18th century, Gulliver's Travels. (Sinbad the Sailor is not much earlier than Gulliver; it's a late addition to the Arabian Nights, though it's based strongly on medieval models.) Because most of the world wasn't mapped, any strange thing could be over the horizon. C.S. Lewis embeds a Fantastic Voyage in a portal fantasy with Voyage of the Dawn Treader, and, because Lewis was a scholar of medieval and renaissance literature, it contains many of the classic features.
Usually, the travel in these stories was by ship, since this was the fastest and safest method of travel at the time (though neither very fast nor very safe by modern standards). However, you could also walk or ride to strange places; the Silk Road, for example, as described by Marco Polo in the late 13th/early 14th century, led to places that were legendary or completely unknown to Europeans. Familiar places and strange places were contiguous, just a long way apart with a lot of tedious travel in between, as they are in most secondary-world fantasy (think about Bilbo and the road that goes from his door in Hobbiton all the way to Mordor and beyond).
Later, as the world became more thoroughly mapped, the possible number of locations for strange places and their strange inhabitants shrank, and became confined to distant, uncharted islands and remote mountains (Samuel Butler's Erewhon, set in the then-uncharted interior of New Zealand; Charlotte Perkins Gilmore's Herland and Arthur Conan Doyle's The Lost World, both set on remote, inaccessible plateaus in South America; Shangri-La in Tibet; etc.). More had already set Utopia on an island in the 16th century, and for that matter, most of the strangeness in the Odyssey takes place on islands. Even as late as the late 20th century, Paradise Island/Themyscira, the Savage Land (hidden in Antarctica), and Wakanda were credible locations, at least for comics.
Gradually, though, all of the Earth was mapped, and the Other land had to be further away. The Celtic Otherworld provided inspiration for George MacDonald, who influenced Lewis, and there you had portal fantasy (as I argued in my previous post). There was the Hollow Earth idea, too, and trips to the moon by various means (starting with Kepler's Somnium and Godwin's The Man in the Moone in the 17th century and leading through to Wells and beyond), and gradually you got planetary romance, with A Voyage to Arcturus via crystal ship, and John Carter projecting himself to Mars by willpower alone, and then Northwest Smith a bit more realistically going by rocket. And then, as we learned more about the planets, that too started to become, not impossible, but consciously retro; when Lewis published his Space Trilogy, he knew that Mars and Venus weren't really as he portrayed them, but he didn't care, because that wasn't his goal. It was a way of putting his protagonist in a strange Other place.
But these days, if you want a fantastical setting with an anchor back to our world in the form of a character who goes to the setting and (usually) comes back, portal fantasy is probably your go-to. Though a lot of similar things can be done by other means; Michael Underwood achieves a kind of portal space-opera by having someone from our world go through a one-way, one-time teleporter from some Atlantean ruins, and I've read some alternate-world-hopping and time-travel fiction that's not too unlike portal fantasy in many respects. Star Trek's planets of hats have a good deal of the DNA of the fantastic voyage in them, too.
And just because we have so many stories like this over such a long period of time, certain tropes get embedded so deep it's hard to see them. And some of those are about the traveler being superior to the weird natives and being able to fix their problems when they are helpless to do so, or the natives being valuable mainly for what they provide to the protagonist, and not being as real or as important or possessing as much agency as the protagonist. We do, after all, like a protagonist to have agency, right? (Though how much agency a Chosen One of prophecy actually has is debatable.) In the Chronicles of Amber, for example, Amber (and Chaos) are explicitly the only "real" places with the only "real" people; everywhere else is just Shadow, and Corwin has few qualms about recruiting an army out of Shadow and leading them to slaughter. This is an intensely colonialist mindset.
But not all the stories are like that, and they don't have to be.
Part of the problem, I think, is that most societies, definitely including contemporary Western societies, foster an unconscious assumption in their citizens that Here is the best place to come from, and People from Here are the best kind of people, and other places aren't quite as real or important, and if they do things differently that may well be because they don't have the benefit of being from Here and knowing the correct way to do them.
This isn't an inevitable part of the genre, though. The early works I described above, for example, where the Other place was intensely dangerous as well as strange, put the traveller at risk and at a disadvantage compared to the inhabitants--though even as far back as the Odyssey, the hero is a trickster who pulls one over on the locals and then leaves.
Charlotte Perkins Gilman's Herland gives us another model, also used in other utopian works (including Utopia itself): the Other place as critique of our society, as a way of speculating about how things could be different. Herland is like Wonder Woman's Themyscira in that it's isolated and hidden from the world (on a plateau in South America, rather than an island), contains only women, and is utopian. A group of men find it and variously are schooled in how to be more civilized/mess the place up because they refuse to be schooled. It's not a great story as a story, but it does give an alternative model.
If we turn to a contemporary portal fantasy like Foz Meadows' Manifold Worlds series, the protagonist goes to a deeply strange and quite dangerous Other place and is transformed by her experiences there (to the point of PTSD). Some things in the Other place are better, from her perspective, some are worse, and some are just different; it's not a one-dimensional comparison. And, while she has some impact on events in the other world, she isn't the sole special Chosen One who saves everyone, and the world probably has more impact on her than vice versa. (I've talked before about the difference between stories where the protagonist impacts the world and stories where the world impacts the protagonist.)
Not that impact on the protagonist in portal fantasy is new either. Lewis's Voyage of the Dawn Treader shows us Eustace's transformation into a much better person; The Silver Chair follows up on this by showing us Jill's transformation into someone who can stand up to bullies, though her and Eustace's intervention is also important in the Other world. I highly recommend, by the way, the series on C.S. Lewis's work currently in progress at Tor.com; it's sympathetic without being sycophantic, and celebrates what Lewis was setting out to achieve and did achieve without ignoring the problems in the texts. There's more to Lewis than his detractors often admit, though there are also more issues than his supporters often admit; this series does a good job of balancing those two perspectives, I feel.
I recently read another portal fantasy in this tradition, the rather lovely (and, in my opinion, funny) Pundragon by Chandra Clarke. The protagonist's main contribution to the situation is actually to mess things up and then do his best to help the (more competent) local inhabitants to fix them; he returns to our world having sorted out some of his own issues and more able to deal positively with his life.
The other big experience that people have of going to a strange Other place is immigration. That's not an experience I have had personally; my great-great-grandparents (and one great-grandfather, at the age of 9) were immigrants to New Zealand, but since they all died long before I was born I don't even have personally transmitted stories of what that was like for them. My wife is an immigrant, though from another Western country, which does make a difference to one's experience (and she looks like the majority of the New Zealand population, which also makes a difference). That's not, in short, my story to tell, or something I feel qualified to discuss in any depth, but I do look forward to reading portal fantasies that reflect the immigrant experience.
Of course, that may well mean that the shape of the story is different. Not "there and back again," a classic hero's journey over the threshold into the Other place and then back again to the familiar, but going to embrace the unfamiliar and then stay there, however much you long for aspects of what you've left. Also, bringing change and enrichment to the Other place from a position of having less agency than the inhabitants, not more.
In any case, if you're about to write a portal fantasy, please think about the tropes, and what they're saying about the Other, before you use them.