Interesting question. In my opinion, the only real chance that Esperanto will ever become truely widespread is if it is adopted by important international organisations. This is not quite as ridiculous as it may first sound. The Grin report suggested that if Esperanto were adopted by the EU it would save billions of Euros per year (I can't remember the exact figure maybe € 67?). So while currently unlikely, it's not impossible that the EU may adopt Esperanto as one of its working languages. Particularly if th UK leave the union. Finally, I have to disagree with the comments about the liklihood of the rise of Chinese as a major international language. From my understanding, China is investing far more into English language learning than others are in learning Chinese. Chinese also has to overcome two major problems. Firstly, there has never been a language so widely spoken on Earth as English. Again I can't remember the exact figure but there is a lecture on youtube on languages by dalana university that is very informative. Secondly, Chinese is hindered by its writing system. This itself hws two major problems: one, the time required for foreigners to learn it well and, second, the Internet and modern technology is dominated by English. This is also intrinsically linked with learning. Major scientific research is always published in English even in China!

When a language is spoken by people from diverse nation, it itself is already called an international language. In my opinion, Esperanto speakers would need lot of time and effort to make Esperanto as important as English nowadays. However, it would be easier to make Esperanto as a complementary language to English, like what we have today, say Chinese and Spanish. We also need to know that when Zamenhof created Esperanto, English (maybe) was not used as international language, and it was not so globalized like nowadays.

Esperanto is an amazing language and has a lot of culture, but it will never be the international language, sadly. Like Jeanne said, the next one will probably be Chinese.

Esperanto will never replace English. The next superpower will determine the next international language. My guess is Chinese.