Toyota is working on a new global BOF architecture. So it's all but certain the next tundra, Land Cruiser, and 150 series replacement will all share a platform. The 150 is likely the last to hit production of the group.
The 150 already has the 3.5 6AT combo in China and other markets. A refresh with that combo is all but certain. If we're really lucky we'll get the 3.5 and 8AT or a turbo 4 with an 8AT. Otherwise - there's no reason to change much of anything. It works, it sells, it's actually pretty cheap compared to anything else out there.
My bet is that the Tundra will get the Cummins 5.0 in the next iteration. In the real next gen 4Runner in 202x the 4Runner gets a mild hybrid and 3.5L ish engine. I don't think there will be more than another 10-15 years of ICE engines in new vehicles. So I'm highly skeptical of any claim of Toyota developing a new diesel that's anything other than a slight tweak of existing ones, or outsourcing. I don't see any reason to spend the time or $ on USA emissions development. The Cummins 5.0 is ready to go with EPA already sorted out. The only small motor is the VM Motori 2.8 used in Jeep and GM Colorado. It's not great motor so far. It was a terrible motor in the Jeep Liberty in the earlier model of the engine. I don't think Toyota would try it.