StarCraft... 3? Ideas for a Prequel.

why not.
Okay, definitely a prequel.

We'll assume that the state of affairs in the current SC2 universe is wrapped up by the time one or two Expansions have followed, after the original trilogy.

So SC3 will focus on the eon of the Xel'naga civilization at its peak, and whatever threats brought them to this Galaxy and necessitated the creation of Protoss and Zerg.

There'll still be 3 races, but no Terrans, Protoss or Zerg at all, obviously. The battles and the entities involved in them would be of an absolutely epic scale; think The Silmarillion versus The Lord of the Rings, with SC3 being like the former; each of the god-like races will have firepower that'd make the Death Star seem like a nerf ball.

In terms of gameplay, it'll also break all bonds with the previous SC (or WC) games, going for different mechanics where applicable, without the obligation to satisfy any existing players' ecosystem, and thus offering true innovation without cannibalizing SC2.

It'd have an "open" multiplayer-capable campaign mode instead of linear missions; each online session would spawn an instance of the Xel'naga's home Galaxy, with the players each commanding a single faction from the 3 races, starting from their respective worlds.
You'd then choose your own path among the stars, managing multiple planets and conflicting with other factions along the way, and occasionally coming up key plot-centric regions which advance the storyline. There'll be different endings depending on which faction completes all their plot-specific missions first.