If we want to select a common language, we should do so based on the criteria that said language is, first and foremost, easy to learn regardless of which language is the student's native language. I contend that no naturally evolved language has this property, which is to be expected since languages are living things with long and complicated histories.

Therefore the obvious solution would be to construct a language with the properties we desire. The difficulty would be in keeping it from evolving on its own and just becoming as complicated as the rest of them.