I'm not an economist but it seems unlikely that India's economy would increase 40-fold in the next 33 years and even surpass the current economy of the world. I have to say, though, I like your idea of determining future superpowers based on GDP.
Economy isn't all about becoming a superpower but okay,these are all likely to become superpowers except Russia and the USA which are already ones,though I doubt Nigeria will get a superpower position in 2050,and same with Germany and Japan,and Russia will lose its superpower status by then probably,also Mexico will suffer quite a bit so it will become a superpower in like...2070?I have a quiz about the 'superpowers' already in 2017.
By 2050 is the definition of superpower going to shift drastically? Because currently it's not merely about GDP. Russia has more of a claim to being a super power than any country other than the USA or maybe China, but it's GDP is only the 12th largest in the world.