There may be recently unexplored areas all around the world, but I see most of them as not worth the challenge. Take central america: Most of the main roads are negotiable by 2WD, 2 or 3 axle trucks. That would leave the headroom we need for TC travel. Many of the lesser roadways and jungle trails drop off fast and are overgrown quickly prohibiting easy travel by overlanding type vehicles. Take the Darien Gap. Several have made it thru this 100 mile swamp, but it could be considered EXTREME in the extreme. I don't bemoan the fact that it is not paved or upgraded. Many see pavement and good transportation as the killer of ancient civilizations.
The other consideration is political. There are many good reasons NOT to get to far off road, even if it is allowed.
Overland was the sub title of my 1949 Willys Utility Wagon, here pictured in Baja near San Quintin in June of 1965. Is that 50 yrs. ago? I"m starting to feel like one of those old civilizations just now. We drove all day @ 10 mph to get to this seaside town down the hot and dusty road.

In N. America, the political factions have slowly strangled any real overlanding possibilities. In my lifetime I have seen many wonderful jeep trails and wild cross country routes crossed right off the map bowing to political concerns. This is much less true in 3rd world countries or countries that have a relatively small population (no matter their geography) as the human pressure on the terra is less and hence less control needed.
I have been all through Baja and it still remains primitive and worthy of TC travel. Hey, it's close (geographically), with a small-ish population of fine indigineous people...... Except along the border.
When I was 22 yrs. old, (50 years ago) the tuba player in our orchestra and I plotted to take an expedition from Capetown to Nairobi by Land Rover. Ah, the draw to a far away place and the ACTUAL Expedition by land rover. Sleeping on the ground and letting the bugs crawl on you; trying to avoid being eaten by a Lion. The smell of ozone. Near the edge. What could possible go wrong? It never came off because we were working stiffs playing 250 concerts a year with the LAPhil.
regards, as always, jefe