I would add a Dwarf faction.
If you think about it, you can come up with a lot of units.
1. Dwarf Warrior (take one from Heroes 3, for example).
2. Dwarf Archer (give him a crossbow and let him shoot with fire).
3. Dwarf on an Iron Dragon (from Heroes 4).
4. Dwarf Engineer (breaches walls) and shoots, for example, with a ballista, or put him on some kind of cannon.

And add a crafting or looting skill to the heroes, so that after each battle there is a chance to get an artifact or resources.