Guess what, he's nobody to anyone when they play War 3 for the first time, he still became arguably the best character in the franchise. That's what you don't understand, you need no prior knowledge for the Human campaign, because it's 100% self contained. The orcs you fight in there aren't thrall's orcs, those already left for kalimdor. You can completely explain it in the first movie. If you don't explain everything it's also great, because you get viewers interested in doing some research on their own, maybe even get them interested to play the games themselves, leading up to WoW ofc. If you go into full detail what point is there for them to do that, they already know all the backstory. No good movie explains everything about it's world just in that movie.
Star was has some of if not the richest lore out there, the movies present none of it, just basic ideas that are needed for those movies to function, if you wanna delve in the lore there are books, games etc. No reason why this wouldn't work for a Warcraft movie.