When Intel designs and makes a chip with the TDP that Apple requires. Even when the MBPr has 32GB RAM as an option I only envision it being available on the 15" version and not the 13".
That mostly depends on Intel. It looks like Intel plans to make 28w quad core processors in 2018 when they release Coffee Lake so there is a chance a quad core Coffee Lake processor may be coming to the 2018 13" MBP model.