Yes, they can. They could add a Janitor class and make it a hero that starts higher than level 1 too. Based on what we know of Janitors in Warcraft, that isn't likely to happen either.
From what we've seen of a Hero class and how they differ from regular classes, their archetypes have been surrounding a seemingly normal class that has gone through some massive change to obtain supernatural powers beyond that of a mortal. That's the bar set for a Hero class. Should they decide to take a Necromancer, which in established lore we know as equivalent to Priests, Mages and Shamans, and turn them into a Hero class, they could very well do it. But why? The Necromancer isn't anything special. Same goes with Bards. They work perfectly as a regular class, as their archetype is already well established in the Warcraft universe. I'm not saying they won't or can't, but to be frank, for them to be a Hero class there'd better be a damned good reason why. And I'm not just talking lore reasoning, I'm talking about their overall archetype and design. These are still in the realm of Core classes in most RPGs.
Hero class is better saved for those special outlier classes that offer that extra umph, like a Dragonsworn that is a champion of the Aspects. That's the bar that is currently set, and I don't think it's likely that Blizzard will lower it just to boost any new class they add to a higher level. They were pretty clear with that when they added the Monk. Hero class is another name for Prestige class from DnD, and Necromancers aren't typically considered a prestige class, just an alternative core.