Well, everyone gets balanced out to be more 'fair'. Protoss vessels are "ridiculously powerful" compared to terrans. A single carrier wiped out a fleet a battlecruisers, in the lore. Yet in a game, they are evenly matched, and will lose if you factor in a yamato cannon.

Well, a few things - first of all, a lot can change in several thousand years. Second, there are reasons they could come across as evil... What if they saw the strife between the races, particularly the havoc they were wreaking on more naturally evolved beings (the terrans) and felt they needed to undo their mistakes? What if, after being released, the voice in the darkness was able to corrupt them? What if the race is truely hitting the end of it's cycle, and now it returns to take advantage of its progeny, by absorbing or possessing them? The possibilities to villainize the Xel'Naga are endless.

Then why would they be a threat to the Terrans? The voice in the darkness is something they already defeated, as such they probably know how it plays, making cirruption even less likely. The millenia old races never just randomly change, in fact they are the most conservative and cosistent of all other races. Picking out a bullshit reason is their best way to villainize them.

The new race will have none of the culture of the Xel'Naga, but possibly a few of the old grudges of the parent races. Knowing Terrans, they will also find a way to piss off the new creations. Especially Megnsk, who might try to imprison the Xel'Naga simply for vivisection.

@Jereel
It is a matter of making the race still feel like it fits the lore. They scrapped the Burning Legion in WC3 over the exact same concerns. That is why I stand behind it so firmly.

Warcraft3 however included the night Elves and the Undead as well though. The is no reason they couldn't have added another race in SC2.

I was just spitballing. Besides, who says they are a threat to the terrans? They only need to be PERCEIVED as a threat for things to get ugly. Besides, they can't just be bad guys for trying to wipe out the protoss? Or there could be one of a thousand reasons the terrans get mixed up with them.

As for old races never "randomly" changing - who says? is that a rule somewhere? What about the Vorlons(or ancients) in Babylon 5? Or  The Protheans in Mass Effect? The ancients in StarGate? And who says it's random? The Xel'Naga were a race nearly destroyed by their own creations. If anything would change your perspective, that's it. Nearly being wiped out, and retreating for thousands of years means we have NO idea how they've changed. Infact, it would be BS to assume they are exactly the same.

Besides, wasn't it Zeratul who feared their return? He must have had a reason...

I mean, at the very least, they played God. They raised up the protoss, and gave them very different sources to draw their power from, simultaneously empowering and fracturing their people. That's about as conservative as Dr. Moreau.

Other than the fact that there is no viable fourth race, and they need to keep it distinct from Warcraft in terms of gameplay.  The only race, Xel'Naga, are against killing by lore (which is required in multiplayer) and suffer from being larger than life similar to the Burning Legion or Titans of Warcraft lore. To maintain the larger than life status would cause game balance to be impossible. While all the races have their impressive units and abilities in lore, they are always portrayed as being equal but inferior to the Xel'Naga save for a few sneak attacks here and there.

Because Night Elves and Undead were so terribly on the table back during Warcraft 2 Beyond the Dark Portal right? They managed to pull 2 races out of their ass, they could have easily done so for SC2 as well.

Do you guys even remember what you are arguing about? xD

I will never doubt Blizzard again...