While a romantic/sexual relationship developing is unlikely unless they are both gay (which it's not like we know their sexual preference but I digress), Hanzo and McCree could likely meet at some point simply due to the fact the Genji was with Overwatch for a bit, as was McCree. In addition, McCree could probably understand some of Hanzo's Yakuza background due to formerly being in a dangerous gang (although they aren't entirely the same thing).

I also feel like their personalities would mesh rather well, or rather what I can perceive of Hanzo's personality through all the "I killed my bro" emotions he has going on, but that's just an opinion I have. Overall, I find it has some merit as a ship but not as much as, say, Mercy and Jack (I'm not really sure whether they are actually fond of each other but they at least know each other from the getgo).

Sorry if you didn't really want to read all this. I don't browse tumblr all that often but I do have a lot of ships and headcanons for various stuff n' things, so I just wanted to give some background on why perhaps some of tumblr ships those two. Then again, most tumblr users probably don't have any real reasons for shipping Hanzo and McCree considering they also heavily ship Jack and Gabriel. THEY FUCKING TRIED TO KILL EACH OTHER WHAT THE HELL. So yeah, there you go, end of rant.