What I mean with Zeratul is that Vorazun already has the whole 'stealth' kit. And to be honest Nerazim heavy weapons don't really fit to me with the personality of Zeratul. If they were going to have Zeratul I would think he would have had the kit Vorazun has
As for Izsha yes she could have a Leviathan hero. I was just meaning that you couldn't have a hero unit of her outside of the Leviathan as she seems to be connected to it.