Just watched the documentary on Netflix about the Vietnam war by Ken Burns (from 2017 I think). The first episode of the documentary covers this conflict (First Indochina war) in some detail.
To cover this conflict could deepen RS2 further, and make possible for different gameplay than vanilla. Perhaps slightly more symmetric warfare, and a bit more like RS1. No need for a new engine. Expansion pack material, without question. Could be great, and as you already said it is quite uncovered ground gamewise, which would make it extremly interesting.