1Engineer, FYI, Chevrolet will be selling a diesel engine Cruze (compact sedan) in the US starting in 2013. Cadillac will be selling a diesel engine ATS (compact sedan, smaller than the CTS) sometime after 2013. Jeep will be selling a diesel engine Jeep Grand Cherokee in 2013.
The current 4Runner has poor fuel economy. Toyota must improve it, due to upcoming CAFE regulations. There are several options for them to do so:
1) Hybrid. Toyota certainly has hybrid experience, so this is a possibility.
2) Smaller displacement, turbocharged direct-injection gasoline engine, like the EcoBoost. Toyota has not been using this technology in the rest of their US lineup, but who knows what the future will bring...
3) Turbocharged, direct-injection diesel engines. Toyota is using this technology overseas.
4) Drop the 4Runner entirely.
All of these options are possible. I don't think it is correct to say that option 3 is infeasible. I do think that there is very little chance of a V8 or a manual transmission option.