Looking at the exit polls and changing demographic numbers, I think Democrats could win it. Maybe even before Georgia as its white vote isn't as Republican (and more elastic?).
I admit I was expecting Arizona to not only trend D in 2012 but swing D compared to 2008 but that didn't happen partially because the white vote swung further to Republicans to offset the Hispanic swing to Democrats compared to 2008[1]. But even then, look at the 2012 Senate race there. That was closer than the Presidential results. I think the 2016 Senate race could be even more competitive if it's an open seat and I doubt 2016 will have Arizona whites swinging Republican again like in 2012.
Basically my assumption is that Arizona will follow the path of Nevada, Colorado, and New Mexico. I don't know if Democrats can win it outside of a landslide in 2016 but I think the margin will be fairly closer than 2012 and if that continues through the next election then 2020 looks like a win, at least on paper.
[1]
http://elections.nytimes.com/2012/results/president/exit-polls
