Going with Metaldog here, I have always said the only way for my son to rebel is to become a suit wearing, briefcase toting CPA member of some Christian Church, listening to country music.
Ewwww Now I have to go scrub that image out of my head.
I’m not a teenager, but I’ve always liked to point out that I’m expressing my individuality by remaining untattooed and unpierced.
I predict more self-mutilation, body-alteration and self-flaggelation. Happily, my kids are WAY past their teens now so I don’t have to worry. THEY’LL be the ones to gripe. I don’t care anymore.
Someone once said that the reason why grandparents and grandchildren get on so well is because they have a common enemy. 
I agree with the others that have said more self-alteration. Just today in the paper I was reading about kids, 15 years old, getting ready to go back to school. Instead of the traditional new outfit or two they’re going all out getting their hair and nails done, getting waxed, going to tanning salons, and getting nose jobs. I had to pick my jaw up off the floor and I’m only a few years removed from high school. This story was in a more upscale community, but that doesn’t make it any less disturbing for me. I suppose it just means there will eventually be a trickle down.
I’m 27 and my mother thinks I am amazingly conservative…compared to her when she was a teenager and young adult, I guess I am! But it’s not deliberate on my part.
Well, they will do it without intentional irony, but the reality will be as deliciously and amusingly ironic as the folks at my college who played lots of Marley on $1,000 to $5,000 stereo systems (ca. 1986).
Experimenting with new and improved drugs. (some homemade)
Unsafe sex/casual sex
More elaborate piercings and tatoos
Lack of concern & knowledge for government and politics that will mold their future yet be outspoken about their “rights”
The ability to burn holes in their pocket and not save for the future
To use filthy language not just as a word or expression in a sentence but as in regular everyday converstaions without wondering if the language is offending others.
Lack of concern for local community involvement
Public defecation seems to be all that’s left. 
I’m only a few years out of school myself, but lots of people I speak to these days are really shocked by the lack of respect school aged children have for their elders. I actually worked as an English teacher of 14 to 18 year olds for a year, and even the nice kids were absolutely impossible to impose any disipline on- they either wanted to work hard and get good grades or they didn’t, nothing I did or said would change that, and this seems to be a growing trend everywhere I think (I was working in Budapest, but I’ve heard very similar stories from people working in the U.K)
Also, the age when kids start becoming obessed with their appearance seems to be getting lower and lower- just today on the tram I saw a kid decked out like he’d just stepped out of a Tommy Hillfiger commercial, and he could’t have been older than 10.
So, as for what these kids will do in future to shock those from the older generation- frankly I think they’ve stopped caring about what we think, and instead become much more interested in the approval of their peers.(with the avertising agencies cheering them on all the way…)
I don’t think you people are using enough imagination. Consider the coming biotech revolution: How boring will a tattoo be when you can grow an extra hand on top of your head?
Please, that won’t shock parents. Every mother I know complains about needing an extra hand or two…the kids would have to line up behind the parents.
Yeah, but what if the extra hand… has a tattoo?
Okay, fine, forget about the hand. How about a dick growing out of your chin?
[Disapproving Parent]So, you want go around being known as “Dickface” for the rest of your life, then?[/DP]
And I’d get suspicious every time he has a stiff neck.
Yeah, ha ha and all… I think the OP is about “gross outs”, not rebellious/shocking behavior. But since we’ve already taken that path…
The web is something that children and youthful adults have these days that kids of former generations really weren’t exposed to, and everyone here knows about the sick fetish phenom so abundantly prominent and so easy to access. Let’s take albino midget bestiality for example (no offense intended to those presently offended): fifteen years ago, this would take more effort to experience than today, when it’s only a few clicks away.
“Ah,” says the accustomed mind of today’s youth. “Albino midget bestiality, once again. sigh Let’s move on to donkey punching.”
Desensitization will continue, but concern for the good will of our offspring will continue also.
It’s just too hard to say what will “gross out” tomorrow’s parents. I don’t think I can be grossed out, and I’m 25. Maybe if the guy next to me had a diahhrea explosion or something…
Funny, a google search reveals no matches.
Not yet. 
I’ve seen four-year olds like that. And I do not think it was their parents’ idea.
Loonies are going to rule the Earth, that’s obvious. Happily, I’ve only got about ten (cough) or fifteen (cough) years to go. Y’all are welcome to what’s in store.
- PW
My friends and I were trying to imagine how boring our kids are going to think our tattoos and piercings are, and how we’ll probably end up saying “But… are you sure you want to dye yourself blue, honey?”
And then somebody said “I bet my daughter grows her underarm hair long.”
The drawing-back “eeeew” feeling from the room was… exactly what I got from my mom when we discuss my tattoos and piercings.
So yeah. They’re going to grow out all that hair I keep shaving off. And they’re going to braid it, and put really annoying bells on it. And wear awful clothing that shows it off.