I'd agree that this is absolutely the best ticket for Democrats in 2020. I'd probably argue that reversing the ticket would be even better. Charismatic, white, representative of the working class, would no doubt flip back the rustbelt, Bullock even being on the side of coal miners. In my opinion the perfect ticket to counter someone like Trump.
Senator Sherrod Brown (D-OH)/Governor Steve Bullock (D-MT) - 348 ✓ ; 53.2% President Donald J. Trump (R-NY)/Vice President Michael R. Pence (R-IN) 190 ; 44.2%And if the ticket is flipped (probably resulting in the oddest 2020 outcome):
Governor Steve Bullock (D-MT)/Senator Sherrod Brown (D-OH) - 377 ✓ ; 53.9% President Donald J. Trump (R-NY)/Vice President Michael R. Pence (R-IN) 161 ; 44.0%I could see a massive swing to the Democrats in places like the rural midwest and West Virginia with Bullock heading the ticket. They'd certainly lack the black voter appeal needed to flip states like Georgia or South Carolina, but it wouldn't be very necessary. Completely new demographic coalition.
Swing Map for Bullock vs. Trump: 
