I agree, engines these days are pushing practical thermodynamic limits so there's not much room for improvement, diesels are already way up there so maybe we'll get a bit more out of gasoline engines but I see it as old tech.
In my mind the ultimate drivetrain is a simple direct electric motor, no parts to break, one gear for all speeds; I haven't done any maintenance on my Nissan Leaf in 5 years. Throw some batteries in there for a hundred km range.
Then throw in a 10 kW generator that turns on to charge the batteries when they get low. That way there is no direct linkage between the gas engine and the wheels so none of that complex transmission crap to break down. The engine works at a single rpm so it is way more efficient and fine tuned, it could even be diesel with better emissions controls at a constant rpm.
This would be the ultimate off-road drive-train, the electric motor gives you lots of fine control for rock crawling if you need to, but then instant huge torque when you want it, plus 0-60 in 3 seconds if you want it. The best of both worlds.
I doubt we'll ever see something like this, instead we'll see hugely overly complex monstrosities like the Chevy Volt that are way too complicated.
But it might be build-able in an old 3rd gen body / frame. Pull the engine out and attach an electric motor to the shaft. Throw in a generator and batteries when battery tech is developed enough for plug and play. I don't see how it needs to be too complicated. The only disadvantage I see for off-roading is if it gets flooded in a river crossing, the you're toast.
