According to Autoblog and Automotive News, Mitsubishi is developing a rear-wheel drive sub-compact for the SE Asia market. Apparently, to no surprise, there was no mention of the U.S. getting such a vehicle. According to the article, the vehicle will be around Kei car dimensions, so it'll be tiny.
What was the last RWD subcompact in the U.S.? I think it would've been either the early 1980s Toyota Tercel or perhaps the Chevrolet Chevette family.