Well if he is one of the Xel'Naga it would be to create the perfect race(that is why they evolved the Zerg and Protoss to begin with)

And well if he is the Overmind it might be to conquer the world to secure his own existance for an eternity.

According to the Brood war Duran worked for Kerrigan becouse he was infested by Zerg, but at the end he just ran away from her and she lost all control of him. My theory is that he let her believe that she controlled him so that he could manipulate her.

Yeah SC is a totally different game in comparison to WC3, WC3 focuses alot on the heroes and different magics while SC is more about the numbers.

Nova doesnt seem to be such an important character so it is doubtful, perhaps as some sidecharacter just to amuse us.