If you read the booklet that gave you the storyline behind all three races, you'd know that the Xel'Naga didn't create the Protoss and the Zerg. They first stumbled upon the Protoss, and found they had adapted extremely quickly to their surroundings. They showed they were technilogically advanced from other surrounding species, and their telepathic abilities only increased their technological breakthroughs tenfold.
However over time the stress caused by the Xel'Naga catalysing their technological progression resulted in civil strife. Fearing for their lives, they traveled to the opposite side of the galaxy to find the Zerg. Though originally independant, the Xel'Naga seeked to prevent individual egos from sidetracking the species development toward racial perfection, hence, the birth of the Overmind.
The Xel'Naga were destroyed by the Zerg eventually (to sum it up), so if they were to be introduced, it would have to be some ragtag force that escaped the Zerg forces. Either that, or inhabitants of their original homeworld. One way Blizzard could compensate for their power would be increasing their unit costs to over that of Protoss units, slower build time, and more restricted building requirements. Zerg need creep, Protoss need Psi... maybe the Xel'Naga have additional requirements.
Nevertheless the Xel'Naga would make a perfect addition to the fray, along with some racial hybrid species going renegade. Protoss/Terran hybrids might not be a radical change, but that could be possible as well.
Speculation can only stimulate our minds, and being able to attain states of deep thought only shows we're more alive than others. Has anyone else thought of any other cool races that could enter the StarCraft universe?