Refering both to the idea of neutral units and resurrecting units; how about instead of a unit that can be used by all races, for there to be two. For the humans, elves and dwarves an angelic hero called the Valkury that resurrects dead warriors and for the orcs, trolls and goblins a demonic hero called the Ifrit that opens a portal forcing cursed souls to reinhabit the dead.
Lorewise, this would work real well in several fronts. The Valkury is a norse myth about angels taking the souls of fallen warriors to Walhalla where they'll feast until Ragnarok begins. Then, the Valkury will take them back to fight the forces of the apocalypse. Sounds like it'll fit with what's happening now. And the orcs seem to worship fire, so Hell might be their heaven and demons their angels.
How I imagine it, but feel free to change anything: The Valkury can either 1. bring fallen warriors back to life from a battlefield for a high mana cost (so if a Thousand humans warriors fell and she uses her skill, a Thousand warriors will rise again. The immortals who were warriors before won't be resurrected though.) or 2. summon a troop of warriors at a lower mana cost on regular intervals without requiring population.
The Ifrit opens a portal that would either allow 1. bring back the fallen of a graveyard all at once like the necromancer but fighting for the living, or 2. produce a constant stream of zombie-ish horde more numerous than the Valkury's but once the portal opens he can no longer move or the overall strenght/h is lower.
And in regard to the moving cities thing + the Goblins being weak (Is it really hard to be green thread), the Goblins already seem to be living in caravans, how about giving them the standard option to move all their cities if wanted? They can't produce troops during the move and it'll be slow, but all Goblin cities have it right away without needing hero cards.