I think it will be the island themes that are already popular now, plus more 'interactive' islands. Stuff like maze/obstacle course islands and storytelling islands (whether horror or otherwise).
The nice thing about the Dream Suite (at least if will work the same way as it did in New Leaf) is that you can set up your island in a certain way to upload it and change the island back to the way you want afterwards. No need to have a huge obstacle course blocking half your island all the time if that's something you don't want!
I would be interested in seeing whether any foreign trends from Dream Suite islands will end up influencing English-speaking AC:NH trends. I know that images and such can still be shared over social media even if they're in a different language, but I would imagine that only a few foreign-language posts would make it into English-language forums and such. Like, are the trends that are popular within the Japanese New Horizons community similar as the ones we see on this forum, for example?