I've owned many built off road/expedition type rigs. My last rig was a 2016 Tundra + FWC. The biggest issue is bigness and weight! I wanted more luxury and absolutely love the Sprinter Chassis. So bought a Winnebago View (one that can run any diesel) with the intention of doing a 4WD conversion. Trouble is, I've driven it nearly 10k miles and the last trip was well over 200 miles off road and haven't needed 4WD. It's been in the snow, sand, dirt, gravel...
Realistically, you get something big and heavy 4WD is less needed. Mine is lifted (clearance), 31" Duratrac Tires (traction), On Board Air -- Good tires and deflating them works magic. Although, I'm looking into a divorced tcase for low range when crawling up hills. I've put the front tire a couple feet in the air... several times. Incredible how ridged the chassis is. I have no doubt I've ventured further off road than most Tiger and Earthroamer Rigs. Plus getting 16-18mpg fully loaded.
Traditional RVs are an excellent option if built with a welded (not wood) framing and fiberglass sections (especially roof). And they are a heck of a value when 5+ years old. There are a good number of quality manufactured rigs. Then add the 4WD or whatever you desire.
I think the real market is in a state of the art hardside pop up.. something with extensive aerodynamics research that is light and can be taken off/put on an existing truck. Gives the consumer more options on vehicle, and they don't have to have a dedicated "expedition" vehicle sitting around. You come in at a way lower price point than a full vehicle, so can reach more people.
The replies to this thread should be a red flag to the op... everyone has a different idea of what an 'expo' vehicle is. Many of the vehicles posted thus far are what I consider 'parking lot 4x4', ie not really going to get you that much farther up the road than a 4x2, mainly due to size, departure angles, etc etc.
I don't think the 'over landing' thing is as ripe as the OP seems to think. It was called car camping for a number of decades. Folks have been cruisng third world countries in Mogs etc for decades. It's been re branded for sure, and has the power of the web, and men with too much money and time powering it (like so many other fields/subjects online), but I'm not convinced there's a huge market out there clamoring for high end rigs.
My suggestion is find the base vehicle you'd like, then take some night classes in carpentry, electrics, plumbing and basic mechanics - then you have two huge plus points - 1) you can build it yourself (and get help when stuck with the build) AND 2) when you travel with it you'll know how to take it apart and fix it.