Tango Bravo

The US Navy is still seeking a smaller cheaper version of it $2 billion Virginia class subs:

"Tango Bravo," or "TB," stands for "Technology Barriers." Whether DARPA can ultimately build a sub that meets its ambitious goal is questionable, but the research could still pay large dividends even if only a few aspects turn out to be successful.
DARPA's Advanced Technology Office and the US Navy will look at ideas like reducing or eliminating nuclear power plant, for example. Since submarines have always been sized around the propulsion plant, these technologies could save many tons of weight and cubic yards of space, thus driving down the required size and reducing its underwater signature.

I agree with the "questionable" part. After the Cold War, the Pentagon was seeking a new destroyer that could do the same things as its huge and expensive Burke class, but was smaller and cheaper. What we got was the even larger and costlier DDX. After the Navy finishes adding so many extras on Tango Bravo, it will probably look like one of our giant Trident subs. The way to go would be to copy one of our allies cheaper AIP submarines, like the Swedish Gotland, mass producing them, then adding extras later.