The fact that Hiluxes and Prados are bare bones utilitarian vehicles is an outdated fact. True up until 10 years ago, but no longer the case.
Why would they merge the Hilux and Tacoma? They're viewed as drastically different vehicles and their markets are very well adapted to each of their offerings. The third world won't like the Tacomas plushness, and the US won't like the Hilux's toughness. It makes no sense. Merging these and losing one of the models would be bad for sales on either side.
The current, top spec Hilux is really comfortable and has all the creature comforts and safety features a modern car has. There's really not much a Tacoma can offer over it except a long bed option and a more forgiving ride. Same goes for the Prado. The current, top spec Prado doesn't even compete with the 4Runner in terms of luxury and creature comforts, it would compete with the GX.
They do offer bare-bones utilitarian versions of the Hiluxes and Prados, that are way lower in spec than even the lowest available specs their US Counterparts offer, as well as diesel platforms. This is where they separate, and why NGOs, miners, agriculture and terrorists like them. But the platforms themselves are already very much cemented and have a lot of coverage.
The Prado and 4Runner share the exact same platform. The same that makes the GX and 4Runner siblings. So there's already a "global platform" there.
The Hilux on the other hand has its own SUV counterpart, called the Fortuner. This one also competes with the 4Runner in segment, and is very much as modern, comfortable and safe as a 4Runner (although I would give the edge to the 4Runner here). There's coverage basically the world over except for the US, where the models clash with the US counterparts. Even then, there's shared powerplants and drivetrain components that would make finding basic parts somewhat easy for people who are travelling, no full-fledged support though.
So that leaves the Tacoma as the only vehicle that has basically no shared infrastructure or shared market with any of the other offerings. It would make a lot of sense for it to share much of its infrastructure with the 4Runner (and subsequently Prados). It would make Toyota USA's life much easier, as well as give it some worldwide support. Making the Tacoma a bit tougher as well probably, but not enough as a third-world work truck though.
In my mind it would make little sense, and don't believe it will become a reality, but it is interesting though.