Multiplayer isn’t currently planned, as that would take too much work for a small team.
Last I heard, their plan is cities that are destroyed to remain that way. The enemy is the only-one able to build new areas. Somehow strengthening cities might get in, but don’t quote me on that. ;)
Thought of another unit.
Undead unit. The spirits of ancient lords, given a burial tomb but still unable to find rest. They have high-quality equipment that has been preserved better than that of skeletons. Their stats are good and they are intelligent fighters, being able to use their gear just as well as they did in life. The armour is fully corporal and can be hit like normal, with no extra penalties to being hit. They are also not able to move around as freely as Lost Souls while wearing their equipment and thus receive movement penalties for difficult terrain. However, like the Lost Souls they lack a stable physical form, so they do not have hitpoints and landing a single hit on their true form destroys them. Their better stats make their true form even harder to hit than Lost Souls, though. If all their equipment breaks they revert to Lost Souls (and gain the appropriate abilities), having lost that what ties their former selves to the world.