I would like a better letter system that can be mirrored without issue, maybe acts as a pronunciation key (see Korean), covers all the human sounds (see phonemes). Not sure what additional grammatical constructions I might add in over baseline English as there are some interesting ones -- Australian Aboriginals have compass direction rather than relative, various Indian subcontinent efforts have different grammar depending upon whether the person under discussion is present or absent.
That said English is the language of science, business and entertainment, and probably will be for a while yet outside of some serious black swan events. Probably would be that, and probably would be that if I had 30 seconds to pick something the nanobot swam would add into everybody's brain.