So, let's start with your last question:
I think the first theme that immediately comes to mind is this: "If I had a chance to redo my life, would I do it?" Portal fantasy is perfect for that, especially if you go the nontraditional route of being realistic about the new arrival's chances in the unknown world. You get extra points for starting the story a few years later from the "portal accident point" and not telling the reader that the main character came from our world until the story's halfway done.
Another theme I suspect would do well is the theme of racism. In a fantasy story, a bully could become a hero, or a worse bully - especially since it's all allowed now at least in certain social contexts. How would the character change upon seeing such social injustice, and would they even act upon it? Will they, perchance, join a nonhuman resistance movement even though they themselves are "human" just because it's right... or will they join the human side because their crush fights on that side?
My favourite portal fantasy(also a fusion fantasy) is a Manhwa called "Id". Haven't read it in quite a while, but it's essentially about an eastern-style martial artist boy who gets transported into a traditional fantasy world but wait someone knows you and you have a talking sword. Fun stuff. Also, his crush is an elf. Traditions should not be broken without a good reason. totally not serious about the last part
The best portal fantasy in my opinion was... hmm... an anime called Re:Zero Starting Life in Another World. The story's a good watch on its own, but its plot twists are amplified if you know the traditional anime/light novel tropes that have to do with the "isekai"-genre("isekai" is Japanese for "another world"). It's a series that plays(and, really, PREYS) on your expectations until you're not sure how the story will end. That's the best way I guess this genre can be written these ways - write honestly, write true to the characters, and let them fail where a person of their caliber SHOULD FAIL. Then watch them fail more, or let them rise up like phoenix from the ashes. The more failures the person goes through, the greater the delight when they succeed.
EDIT: I am sorry for the long post. This topic is something of a passion of mine.