Out of practicality, this should be English. English is one of the simplest languages and a lot of people already speak it. Things based on history should be out of question entirely, be it Latin or Greek or whatever the people before suggested. Those languages are completely impractical and not used anywhere.

Esperanto has failed to gain a substantial amount of speakers. I think if humans ever choose the path of non-natural languages, such language should be developed with deep understanding of how human brain works, to extend the mental capabilities through such language if it is possible - we're simply not there yet in research.

I also think that there should be a possibility to opt out of learning the national language - that is, teach English and conduct all education (remotely) in English and make so that it does not become a barrier when living in EU. Right now I can imagine the education part, but life would be impossible in most countries within EU, especially in the east.