|Protoss as they appear in Starcraft II|
For the Protoss and the Humans to have combined forces and perhaps even hybridized their races would require some rather radical changes in the all-out war between them circa Starcraft II, but if the Zerg were to discover a way to infect and destroy entire planets from the inside-out, perhaps such an alliance would be born out of need?
But what of Humans in Warcraft? The lore within the game suggests that they were descended from Vrykul, a human-like race of giants, but that humans were the weak offspring of Vrykul who refused to cull their young as their leaders commanded. What if this was no mutation, but a reversion to type? What if the Vrykul are actually descendants of ancient (very, very ancient) humans from the Starcraft time period? That could explain why these mutated offspring breed true. It might be that Vrykul have to kill off their human progeny in order to maintain their relatively recessive, non-human traits (most of which appear to simply be their size and cold tolerance).
Ah, I hear you say, but what about magic? Well, there are several kinds of magic in both Starcraft and Warcraft. The Protoss have psionics, the basis of which was never clearly explained. There's also a lot of Clarke-esque "advanced technology indistinguishable from magic" going on in Starcraft, especially in the Protoss camp.
Another interesting angle is the Xel'Naga, a race from Starcraft which is reported to have manipulated the development of both the Zerg and the Protoss, and which will play an important role in future developments for the game... If those two races gained access to different aspects of the Xel'Naga race's capabilities, it's possible that they each found a way (of untold thousands or millions of years) to use that advanced technology to create "magic" in the time of Warcraft.
We know that Demons are, in part, the creation of one of the Titans, gathered from various races starting with the Nathrezim from many different worlds. We know that "The Light" appears to be code for manipulating some sort of power source, be it of religious, natural or engineered origin... that much is never made clear. The Naaru are attuned to it, and were able to connect the Draenei to it in a way that Humans appear to have had a natural affinity for. But there's also quite a bit to support the idea that while Humans see The Light as a quasi-religious phenomenon, it's not quite that simple.
So, now that this new MMO is coming and Blizzard is coyly describing it as "new IP," rather than "original," I think it's clear that there's something going on, here. Will the new MMO take place a few thousand or million years after Starcraft and focus on the origin of the Titans? I think so. We'll see...