Short answer is, apparently, I wouldn't; or else I'd have to hold really good open casting calls, because I could not think of a single actor I considered right to play Thor.
But I did have some ideas for some other Norse gods: I want Tina Majorino (Mac from Veronica Mars) to play Hel, because I think she would rock that role like an unspecified rocking thing. I want T. J. Thyne (Hodgins from Bones) to play Loki, because he has the exactly right manic chaotic spark, and also it would fit and be just right that he would be fairly small compared to the rest of the gods. And my moment of brilliance in this, in my own completely unbiased opinion, was that Tahmoh Penikett (Helo from Battlestar Galactica) and Jim True-Frost (Prez from The Wire) should play brothers in lo these many things, and in this case Tahmoh Penikett should be Tyr and Jim True-Frost should be Hod. I'm also thinking of Allison Janney (C.J. Cregg from The West Wing) as Frigg.
Anybody else full of theories? I know, I know: startling lack of Jane Lynch. timprov thought she should be Hel instead, but I just can't give up Tina Majorino in that.