I feel like fanboy-ing a little bit here. I've compiled a list of bands that have either never come to Canada (as far as I can tell) or who come here very rarely and I have never seen. If any of the bands on the list see this, come to Canada! I can promise that you'll have at least one ticket sold and that I'll do everything in my power to make it to the concert.
In semi-alphabetical order:
...And of course Wintersun can come here as many times as they like, I'll also be sure to buy a ticket every time to see them. They're the only band that I can guarantee I've seen every single time they've toured in the country.