Any race can be any class in World of Warcraft. That is, there is no ‘physical’ limitation that prevents any race from being any class. Forsaken can wield the light, and Blood Elves can call nature magic. (Both of which we see NPCs do) There is nothing stopping an individual of any race from being any class.
That being said, I do acknowledged that players are not individuals, we are the masses. The race/class combos that exist represent what is common for those groups culturally. You can’t play a dwarf druid for the same reason you can’t play a dwarf that shaves his beard off. It’s not a part of Ironforge’s culture to be druids or shave.
Now, lets be honest, some of the race/class restrictions don’t make sense. Limiting hunters and rogues doesn’t make any more sense that limiting warriors. I don’t personally understand why any race would have ever been restricted from those choices. With Legion, it looks like hunter will now be available to everyone. Rogue should too. There are several Tauren pirates out there. Tauren pride themselves on being hunters, and being stealthy is an important part of hunting. We’ve also seen Draenei in SI:7 and a race that has been on the run and in hiding for ~25,000 must be good at stealth right?
Each expansion opens up new story opportunities. Adding new combinations is a great way to show the progression of each culture in game. The best example of this is when the Wildhammer and Dark Iron dwarves moved in to Ironforge. It expanded their lore and their game play options. It would have been nice if more story had been done when monks were added, rather than a Pandaren plopped in to each starting zone. I just rolled a gnome monk and I feel like the characters has no direction. Once I hit level ten and was able to be a brewmaster I felt a little better, but honestly, at level 10 what does my gnome know about being a mistweaver, or a windwalker? I only talked to Xi, Friend of the Small once, and all she said was “Go talk to Mekkatorque.”
Race is little more than an aesthetic choice. I personally would like it if Blizzard removed the race class restrictions, but I would want to see them do it in a way that is interesting lore wise.