I don't know, but I'd rather prefer French or German to become the lingua franca of the EU. Why?
Because those two languages are spoken by a large amount of population (actually German is close to 100M speakers, that's close to 1/4 of the whole population. French has fewer speakers, but is probably more studied in Europe than German.
Also, those two nations, in their own unique way, are at the very core and center of the EU. Both believe in it, both want it to keep going, thriving.
Call me crazy - but I'd even prefer Esperanto being used as the official European language as compared to English. At least the EU would not be giving an "easy" advantage to American and British citizens, just by the fact of being native speakers.
It just wouldn't be fair to have European kids and adults have to learn a minority language while Americans or Brits would just get it for free.