Yes, an infested archon would be nice......although I can't see it happening.
I personally would like to see a plot twist. Maybe all the Xel'Naga were not all decimated and the remaining ones were watching the events of the Starcraft Universe from the deep recesses of space. :-/
Also, Duran might be a Xel'Naga in disguise, cross-breeding the Zerg and Protoss to create the perfect subspecies. (In the history of Starcraft, you learn that the Xel'Naga made the Zerg and Protoss to try to make the perfect race.)
From that point, the secret mission makes a bit more sense. Duran would cross-breed the two minor species to make the "hybrids". They would have the intelligence and psionic prowess of the Protoss, and the voracity, adaptability, and single-mindedness of the Zerg.