1. Goblin Marauder – as suggested, let them throw stones with some kind of effect.
2. Orcish Riders – put the same orcs or goblins on wolves with a "corpse eater" ability.
3. Mountain Warriors – the base could be any orcish unit, simply recolored, with two hammers or axes.
4. Lords of the Earth/Guardians of the Earth – a mage, only male (take the Assassin and instead of an axe, give him a staff with a skull, and modify it).
5. I don't know yet.
6. Borgs – take unupgraded Death Knights from the Necropolis faction and put them on dark dragons (I've even seen a model like that somewhere), creating something like the Fellbeasts from Lord of the Rings.
7. Trolls or Yetis – both live in the mountains.
P.S. Another idea: you could change the catapults, like in Heroes 3, where wyverns shoot. Let cyclopses shoot instead of catapults. To me, that makes sense; a catapult throws stones, and a cyclops could easily do the same