Well there was the C-17B proposal with more powerful engines, strengthened structure and an extra main gear bogie in the fuselage between the existing gear units, engineered specifically for short, rough field performance.In decreasing order of probability, some being pretty much zero -
4. Buy C-2 from Japan or build it under licence.
5. Americanise An-70 & build it in the USA.
6. Revive C-15 (note that your 2 is reversing the process which led from YC-15 to C-17).
I've discounted KC-390 as its only a very little bit bigger than C-130.
Then again they could always revisit the YC-15. The original design is way too old now but it is the concept that led to the C-17, I wonder how long it would take to develop an A400 sized aircraft with the same configuration as the C-17.
How about the KC-390?