I don't think there really is a Canadian designed cartridge of note besides the .280 Ross. But there were several developments in hunting bullets invented by the old CIL company that were really innovative and a true advancement in ammunition design. Then the Americans grabbed onto the ideas and claimed the glory as usual.
The Copper Point Expending bullet was a very streamlined bullet for long range shooting. First of its kind. Subsequently copied by Remington as their "Bronze Point"
The Sabre Tip was the first of the pointy plastic tip bullets that were commercially successful. Norma may have marketed their plastic point a little sooner, but it was a blunt tip. Imperial ammunition got the design right for a high ballistic coefficient bullet that does not deform in magazines under recoil. Then it was copied by Nosler and they bragged up their invention of the Ballistic tip. Now all the American companies make plastic point ammo.
The "Kling Cor" Soft point was also a great innovation. A traditional soft point jacketed bullet with the addition of four divots in the jacket that locked jacket and core together so that they would not separate on impact. Hornady later invented the new "Interlock" Sure they did. Really.
Oh, and I lately noticed the "US patent" mark on the Federal ammunition 10 round plastic belt slide. Federal certainly didn't invent that. The originals were yellow and looked otherwise identical, and were first marketed by CIL Imerial ammunition in the 1980's, a decade or two before Federal ever got the "idea" for their patented "invention".