One trope that is doing better and better in JP and might come over is the: "I'm reincarnated but the world is at peace"-trope. Followed by the usual "I'm summoned but I'm not the hero/villain" which already has gotten a lot of traction. Before that, we got the whole "OP but only because it's X really good".
Based on those, I would expect more tropes that subvert the current tropes without destroying the overall story. Or settings (sci-fi, historical, etc) that subvert the usual "tolkien-esque" fantasy of isekai.
Another rather easy thing to predict are changing game genres for isekai. We mostly have (MMO)RPGs, but there'll be that "first big" strategy/shooter/etc story that'll create a rush of those tropes. Shooter is already kinda happening, but managing/strategy might become the new "low-level kingdom building". The new version of the dungeon systems.
Another interesting thing to look out for would be stuff that borrows from classic thrillers. In traditional publishing, the success of that genre is tied to the unrest and growing uncertainty in the real world. In that regard, I could see tropes that borrow from that more traditional stories and use arcs or themes from those. Given that a lot of entry-level writers train with webnovels, I don't see a full switch to thriller/crime, but I could see tropes that also discuss uncertainty.
Or - to "battle" the real life - we could also see more "perfect 'little personal space'/family in a hostile world". The really grim and dark world but the main character actually doesn't do anything and just lives their live. Kinda in that same anti-direction are also the "I'm the villain that does good things in a bad world" settings that pop up more lately. The "resistance"-setting without the "Hunger Games stink".
Another thing we'll probably see more of are "systems without reincarnation", where the system is part of the society. Something that takes the "gene"-formula and mixes it with the "LitRPG"-formula. I think we already saw the start of that with all the "I cultivated in a game and now I'm super strong in the real world". Already popular and they might become dominant with some changes to make them less "magical/unlogical".
Since these stories aren't very original, it makes sense they didn't invent any new tropes that didn't exist in the world of fiction already.
That's true. It's probably more a question of "What old tropes weren't popular over the last 10-20 years and will come back with a new coat of paint?"
Will we see the next era of the "single boy in an all-girls X"?