Being King of Canada would not replace the Prime Minister. It's like England, which has Parliament and a Prime Minister, and several political parties. Parliament has more law making power, and power in general, than the Queen or King.

The Governors General in Commonwealth countries represent the Queen (or King) and their signature is required on a bill once it passes the House, then the Senate - but by the time it passes those two houses, the signature is pretty much a rubber stamp.

If Wills and Kate take the throne, they just might have more interest and a refreshing perspective on the whole thing. It'll be interesting to see. Charles, next in line, is only 62 and both his parents have lived to a good age. Unless he passes the throne to his older son and is an advisor to him, it could be several decades before we have a King William.