

Chinese, Korean, Vietnamese, and Japanese are totally unrelated languages. Chinese languages are sino-tibetan, Vietnamese is austro-asiatic, Japanese is japonic, and Korean is alone in its own family. Totally unrelated to each other as far as we can trace.
Despite that, they all used to use the same writing system and, shockingly, they were mutually intelligible when written down. In Japanese this method of reading Chinese (without actually knowing Chinese) was called kundoku but I think that the other languages also had ways to read & write Chinese writing with very light translation. Even today, Chinese writing unites the different dialects/languages of China.
My proposed lingua franca is the Chinese writing system. Everybody should keep their own writing systems, but they should also learn to transcribe into Chinese, the only extant written language in which this is really possible.














China has an extremely high literacy rate, so the difficulty in learning the system is, at least, provably surmountable.
The strength of being able to unite communication historically across East Asia and potentially around the world is a pretty big plus. Offering such a strength impossible in other systems, ideograms are hardly equivalent to imperial units.