Which countries will be major world powers in 2050

A lot can change in 32 years, but I would guess this, if you mean in a military sense:

Superpowers: USA and China.

Great Powers: UK, France, Russia, Germany, India.

Emerging Powers: Nigeria, Iran, Vietnam, Australia.

And in an economic sense:

Superpowers: USA, China, European Union (led by Germany and France).

Great Powers: UK, Japan, South Korea, India and Russia.