I could see the 4Runner going to unibody, but otherwise "designed" for offroad with an elevated body, approach angles, all the 4WD bells & whistles, styling, etc.
People here say that if the 4Runner went unibody, it would be the same as a Highlander, but that is simply not true. Toyota can keep all the same offroad goodies and add more, while maintaining an aggressive and masculine appearance that is very separated from the Highlander. While the Highlander is built for comfort and refinement, I'd expect the 4Runner to still be purposely utilitarian and somewhat truck-like.... but with unibody.
The purists of course will protest, but eventually there will be no other option, and the vast majority of 4Runner drivers will find that the 6th gen unibody will be able to do everything they do now, probably better, while towing the same, and in more comfort and with increased safety & efficiency. And that in itself will pick up even more 4Runner drivers.
Except almost every Unibody Jeep (Chry-crap can call it whatever they want, it's still a Unibody regardless) owner I know is having issues with the Unibodies cracking and warping. The only guys I know that aren't having to weld their unibodies back up after decent wheeling are those that have either built Exo-Cages or built dedicated roll cages and reinforcement structures that you would never see on a mass production vehicle due to crash rating issues and such.
I could see a unibody design that retains the same or very similar running gear including the solid rear axle. i mean look at the jeep XJ Cherokee, or the grand Cherokee till 2004. both models were unibody designs with solid axles front and rear, and both get hardcore guys that beat the piss out of them on 1tons. just because there is a possibility of the next gen runner going unibody doesnt mean its the end of our offroad mods. we may just have to get used to adding stiffening to the structure.
Except almost every Unibody Jeep (Chry-crap can call it whatever they want, it's still a Unibody regardless) owner I know is having issues with the Unibodies cracking and warping. The only guys I know that aren't having to weld their unibodies back up after decent wheeling are those that have either built Exo-Cages or built dedicated roll cages and reinforcement structures that you would never see on a mass production vehicle due to crash rating issues and such.
At that point you might as well buy a Highlander or any other crossover and do the same, because that's what it comes down to really...
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It's a safe bet that the 6th Gen will carry over the J150 Frame (as far as I know there is no new Prado Platform planned before/on 2017) just like the 3rd Gen Tacoma did. At this point the 4Runner will basically take much of it's cues from the Tacoma and besides being a slightly updated J150 Frame (the body mounts behind the front wheels will be beefier and wider like the 3rd Gen Tacoma) it will be a new body on the same frame much like the 4th Gen to the 5th Gen was. Still an improvement, but in terms of the general platform a relatively small one (which actually will work out will I suppose as both the 4th & 5th gens are hitting a good amount of aftermarket support that will theoretically need only minor adjustment to work with the new generation vehicle.)
The theory that the 4Runner will adopt the GX as a "Premium Trim" is an interesting idea; but ultimately it rings more of wishful thinking than probable reality. The main reason these vehicles are getting axed/downsized/moved onto weaker platforms (at least from a off-road/work capability standpoint) is due to fuel mileage, emissions, and safety standards. Toyota is moving towards smaller engines, with forced induction in the near future and as a result they are trimming the "fat" so to speak from their line-up. The Sequoia may not live much longer after 2016, it will depend more on sales I imagine as many families are opting for Siennas or Highlanders vs. the big gas guzzler. I imagine that's partially why the TRD Supercharger program was dropped, many people weren't spending the money on very expensive kit that further reduced their gas mileage; there's also the fact that Toyota has had some issues with their engines not handling the additional boost well in a stock format. That's something that could easily be fixed by changing out just the connecting rods (they tend to be very thin and light to reduce rotational mass, decrease weight, and increase fuel economy ultimately... are we seeing a theme here?), but that would add an additionally expensive cost and labor to a relatively simple "bolt-on" kit.
The Tundra and Tacoma will obviously stay for a long while, Toyota has put a lot of money and effort to be in the "Full-Sized" truck game and it would be foolish to try and pull out. But I imagine that you'll see an all new Aluminum Body in the next gen if Ford's gambit pays off in the long run; possibly a V8 with D4-S as I imagine they need to really bring up that thirsty mpg rating.
The Tacoma is their bread and butter in the Truck segment (so much so that their competitors are finally trying to fight back) and while it has a slightly softer side, it is to the US what the Hilux is to the rest of the world. I would imagine though that to simplify production and development, they may try to consolidate the Prado & Hilux Platforms at some point in the future to better maintain a BoF presence if they decide to go that route. Though at this point the more rugged Hilux is also sold at a cost point, so if they could line their pieces up to make a global platform convergence like that easy and profitable I imagine they would have done so years ago; but as pressure to decrease BoF vehicles increases it may be the route they go.
Diesel options for both the Tacoma and Tundra will depend mainly on market demand, emissions regulations, and profitability. Toyota has made it clear that they believe converting their existing diesel platforms to meet US requirements would be too costly with little profit return. If GM's little Canyon & Colorado sell extremely well in the Diesel trim, Toyota will likely change their mind; additionally it seems their latest batch of diesels shouldn't require as much modification to make work in the US. So there's that hope I suppose?
In the end, the 6th Gen 4Runner will be a BoF vehicle mainly based upon much of the existing technology showcased in the 3rd Gen Tacoma. It does seem that for all intents and purposes Toyota is keeping the rough Prado platform dimensionally the same for over 12 years. I am not entirely sure if that's because the J120/J150 frames are "just right" and need only minor tweaking or because Toyota wants to stretch the platform a little longer so that when the Hilux Needs a new frame they can combine the two into a singular global platform. Where the 4Runner goes after the 6th gen, well time will tel. Supposedly due largely on new technology and development methods the development process for a new vehicle is somewhere in the 30~36month range (down from 4yrs) as long as the rough design is well received at public shows (if it's not, some changes may be made to make it more appealing), this is down from the older 4~5yr period. I imagine that if they are also designing a new drivetrain from scratch that period might be a bit longer than 36months (one source said that it was as short as 18months for Toyota, but I am not entirely sure I believe that...) The point being here that the 6th gen will basically be the proverbial frog in the pot, given a few few into it's release I'm sure Toyota will have decided if they are going for a 7th gen or if it will be scrapped.
We could go on and on speculating, but I think the above synopsis is probably closer to what will happen as it matches up with Toyota's current goals and previous history. Though it's all speculation so they could make a complete 180 tomorrow.
I do think that they should make an effort to retain a BoF vehicle line, an obvious choice would be to converge the Hilux, Prado, and 4Runner platforms into one Global BoF "Land Cruiser Jr." Platform. This would relieve resources in development and production as they would have 4 vehicles that share a lot of common components, drivetrains and features could be adjusted for various markets easily (i.e. the Hilux & Prado may have a more "stripped down" feature list and engine choices for a lower price point in less economically developed areas that have lower emissions standards, while the Tacoma & 4Runner continue to cater to the "Upscale" North American market). If they really wanted to take the commonality further they could just rebadge the new "Global LCJr" platform in much the same way the 4Runner and Hilux Surf were exact duplicates with different names, that would mean 2 less platforms to have to develop fully vs. simply adjusting 2 existing platforms to meet global needs. It would be a logical way to trim down costs and resource drains from retaining a BoF platform Globally & in the North American market, as well as continue to rake in profits by holding out in the "rugged, capable, BoF" market segment that is quickly dwindling.