There's little doubt that it would be whatever English evolves in to. With 1/3 of the world's population using English as a second language, the center of gravity no longer rests with native speakers of English.
I am skeptical that it would happen so quickly - just 100 or 200 years into the future - but that's possible. Communication is a practical thing. People will communicate in the way that best serves their needs.
Even though I'm a language lover, I'm not sentimental about the loss of languages. Who knows how many have been lost across the millennium that we don't know about. The world is richer when a greater diversity of people can communicate.
Also, there will likely be people who still study and use their language of heritage. However, they won't be 'native speakers' in the true sense of the phrase.