Personally, I would love to see Bullock, but according to our resident MT commenter MT Treasurer, he is not that moderate and he is not that popular and he would not be such a good candidate.
I'm not really on the Bullock train myself, but one note of potential disagreement here...
IMHO, not being that moderate actually makes him *more* likely to be the nominee than if he were a bona fide moderate. John Edwards 2008 had the right idea, I think (I mean in terms of campaign strategy, not in terms of personal life
): The default assumption from voters in a Democratic presidential primary is that a white male with a Southern accent is going to be the "moderate" option in the field, and there just aren't enough Dem. primary voters that that appeals to to make that base the sum total of your voting constituency. So you've got to branch out and move left on policy.
Same logic applies to Bullock, being the white male governor of Montana. Default assumption from voters will be that he's "moderate", but if he's pigeonholed as the moderate in the field, then he has no chance of being nominated. So he'd have to expand his base beyond that.
Now, it probably won't work, as it didn't work for Edwards either, and he's a longshot to be the nominee, but in both the Bullock and Edwards cases, it makes a heck of a lot more sense than running as the next Joe Manchin.