Not encouraging this but a merged US-Canada would be the most powerful political entity in human history.
Here would be some of the overall stats.
- Population - 380 million
- GDP - $30 trillion
- Land area - 19.5 million km2 (4th largest in history)
- Mineral value - $78 trillion (more than Russia)
On top of that you'd have the largest military by far on the face of the planet and a global reach to match it. You'd also pretty much have every resource you could want within your borders so ironically enough this new nation could theoretically engage in protectionism.