So likely something that is theoretically useful or practical in some way but ruined by bastards. I would try for the pure fad/novelty options but if I could predict those I would be doing far better. Mobile phones have probably killed any chance of another tamagotchi, and despite emulation existing for many decades I am going to have to live with the likes of the NES classics of the world. That said N64 mini is a harder sell on many fronts (the N64 was not exactly successful, the licensing if going to be a pain, it looks likes arse today and technically it is no mean feat where a NES emu is trivial) and that leaves PS1 mini or maybe a competent sega mini. Reservation held in case they combine the NES, SNES and others of that vintage.
There has not been a massive advance or price drop/patent expiry in batteries, motors, computation*, materials, anything that might fly, input methods or similar. I can't rule out a mixture of those into something new but most of that has already happened and we are a while off human type flight. Electric pushbikes could be a thing but UK wise there are already fairly serious laws governing those.
Trends going in cycles could be another. Ignoring fashion I am not sure what the late 80s and early 90s brought us beyond cartoons becoming even more blatant adverts and censorship lessening a bit too. Also under ignoring fashion is whatever the "I'm bringing it back" crowd bring back next like they did for watches, walkmans, polaroids and such.
*it was possible to have a fancy active steadycam 10 years ago but today when a micro and a cheapo ARM can pull it off we get nice gimbals we are seeing today.
Fads in mobile phones software then... if pokemon go had been a vaguely good game, never mind actually being the pokemon real world mmo/arg you know will make a packet for someone one day, it would still be being played. Your candy crushes and farmvilles are mainly ragged on for their shady business practices. The IR camera addons for them are still too expensive to warrant them being used to look through someone's top by all but the most dedicated (or those that remove the IR filter themselves). Maybe a wearable item that does some combo of streetpass, matchmaking and some other kind of vaguely "social" thing you can think of to make a VC fund blow their load in the first meeting. Someone here will make a comparison to kinstones on the GBA Zelda Minish Cap and it will become something of an in joke.
"Advancements" in vanity... I can already broadcast stills and video of my bowel movement and nobody wants that in smellovision so probably not that. Others mentioned follow drones and flight is still that little bit too tricky to do by AI in a vaguely affordable device. Give it a few more years though. Camera glasses have failed again it seems and will return around the same time 3d/vr goes portable. If a gimbal based head camera thing for a gopro gets made and is somewhat cheap then that maybe -- loses head wobble and if I turn my head sharply it negates that somewhat. If nothing else that would probably up the quality of youtube somewhat.
The topic says next so I am not going to call some kind of neural network/AI based affair just yet but wait for those to come. I am guessing someone will combine some kind of optical recognition make up removal application with a horoscope (and a freemium model of course) to create a phrenology for the 21st century, however that would have been the obnoxious and vapid that go in for that anyway. News articles are written by bots and before long I reckon some kind of auto video making social meeja platform will appear to combine conventional text and photos into some kind of inane automatic video, possibly with automated voice of the person in question if they can train it well enough -- vine but even less effort. Pending that all social networks are the same core archetypes these days and thus as disposable as instant messenger protocols.
We are about due for a semi educational fad and if the 50 different paid educational platforms advertising all over youtube
If I am similarly out there then ultrasonic speakers -- they exist now and are fantastic but you are not going to get one in a beats pill/something I might mistake for my dog's chew toy type thing any time soon. Before that you will probably have someone combine the wireless headphones apple reckons we should all have now with a flash mob and a mass broadcast by some future iteration of bluetooth to see videos of some borderline oxygen thieves dancing like they mean it in apparent silence in a town centre somewhere.
Taking it back to the neural network thing then I see they are better lip readers than humans now (for well lit front on shots anyway). If the mosquito noise taught us anything then that will be a fad somewhere in communications. Free name for any such people -- voxless.
If the food companies doing good science hire a marketer worth paying then they could get something good going on a la Betty Crocker once they added the need for the egg. Instead snake oil salesmen and organic evangelists will probably continue to peddle nonsense and that will be the bulk of that world (though if you can sell old junk to people and not have to do good science then makes sense from a business standpoint).
Hacker conventions see laser light shows being popular. I reckon some probably Chinese maker (because safety regs) will combine a laser pen with a game (get an optical sensor in on it, indeed I had something like it in a shine lights in the room and shoot it toy a decade or more ago) and a light show of some form. The kids will then all be shining their laser on the wall to play some simplistic game (or simplistic game that they can customise/grind in).
If wireless charging/power transmission or seriously rapid charging (seconds for at least minutes of use) gets cracked that changes things. The ability to wear a reasonable power pack and have it power a lightweight device nearby (1m radius, maybe 1.5m) would change a lot and allow for something like the wearable glasses to record everything and overlay cool info. Combine it with some kind of active resistance charging from clothes and it only gets better.
Body modification. I don't think programmable/moveable tattoos or ones only visible under UV or something will hit mainstream just yet. Everything else is fashion and I already dismissed that. I don't know if "useful" modification (adding a compass to your body or similar) will get big any time soon.
Making stuff. 3d printers ran into the problems most with even a vague understanding of the concepts involved expected them to run into. I dare say the home user pick and place machine set is going to be running and hiding.
Waffling now so I will leave it there.