European Union. Plain and simple:
1) most stable government,
2) most peaceful region,
3) already the world's largest economy.
China? Only if:
1) There are no revolutions or coups or threats to the CCP's power.
2) There is no war in the Taiwan Strait.
3) Reforms in both politics and economic structure to address: A) Social Injustice, and B) Widening gap between rich and poor.
India?
The whole "India and China" thing is really a figurement of the western imagination, the only advantage India has is an English speaking education system. The foundations of the India economy is much weaker than China, and India government's power and influence in the world is a joke compared to the omnipotent CCP.
Russia? Short answer is no. Long answer is maybe as a major power in the Shanghai co-operation organisation, but not as a superpower on it's own.
Brazil? Too corrupt, too much injustice, no chance of reforms.
Canada? If the entire planet adopts Nuclear power, and the middle east and central asia becomes real chaotic, then considering how much Uranium and oil sands Canada has...... It's not that impossible.