Some sort of game akin to Civilization or Dwarf Fortress (Or some hybrid thereof) where your goal was to, in a land of humanity and the various other fantasy races, take over the world using necromancy.
Too few games end up taking a large scale view at the consequences of being able to raise kills, whether freshly slain or long dead, to serve your own army; an army that doesn't need food, water, sleep, that never tires, never complains, and never breaks down due to lack of morale would have a huge advantage over conventional forces.
You would have to work in secret at first, raiding crypts for old weakened bones, and possibly artifacts or secrets to stronger undead. You would then start small, amassing your undead army, with each new kill a potential recruit, whether to your army or the village you just took over. After you reach a certain point or level in power, you have to make the decision of whether you allow your thralls to engage in a limited form of autonomy, risking a rebellion but increasing your maximum control due to not having to concentrate on each individual, or ensuring absolute obedience while stripping them of their free will, resulting in smaller but more potent forces, and possibly driving you closer to the brink of insanity from the stress of keeping multiple souls in your grasp.
Battles would be real time, akin to the Total War series, but with differences in how you command your troops. Given the choices above, do guide your armies in the right direction, and hope the intelligent undead are able to direct them successfully? Or do you have absolute control over your troops, meaning that you must micromanage each group of minions in such a way that would make 200+ APM Korean Starcraft players cry?
There will be as much focus on the individual as there is on the whole as well. You are a powerful sorcerer in your own right as you grow, able to turn the tides of a battle on your lonesome, but at the cost of your undead minions. Human resources takes a new turn when you must sever the connection for some of your forces in order to cast some of the higher rank spells, and some of them will not be happy about getting cut off early. You must choose whether to focus on offensive spells, such as the basic fireball or shadowbolt flinging, defensive spells, such as magical circles that strengthen your troops fortitude when stepped in, conjuration spells that can bring in powerful yet limited time troops from other planes, or simple necromancy, which frees you to raise the dead mid-battle and bolster your forces further.
As you go on, you will need to train apprentice necromancers to extend your grasp, with randomly generated characters using a light RPG stats system, with a multitude of hidden statistics affecting their choices, similar to Crusader Kings 2. While they are your apprentices, they each have their own goals, ranging from absolute loyalty to planning to overthrow you when your back is turned. You will, of course, have ways to deal with them as well; UristMcNecrobackstabber won't be so uppity when you have the magical equivalent of a bomb planted in his chest, ready to activate at a moment's notice.
There will be a tech tree of sorts, as is custom for civilization-esque games. Do you go the way of the spirit, with incorporeal undead riding at the forefront? Do you spread a flesh eating plague among your minions, causing them to fester with pus while inflicting terrible casualties among your living enemies? Do you want a basic, no holds barred zombie apocalypse, with self propagating minions? Or do you call upon the secrets of the void, causing your armies to twist into eldritch horrors from beyond the stars, causing terror and insanity in both the enemy and yourself?
Do you reign through fear and terror, or do you fight for the right of the dead to enjoy their everlasting peace?
Wrote this basically before I went to sleep, so I hope most of it is coherent. If nothing else, it's something I would love to play, if not make myself, but I wouldn't even know where to start should I want to attempt programming a game of this scope, at least in terms of viability. While I'm a programmer, I haven't tried anything all that big before.