Here's some ideas I got while on the bus:
- No fancy Interceptor or Skyranger for you. Have fun with some old chopper and two (old) Super Tucanos that Embraer was very kind to loan to you, because they're making INCREDIBLE ammounts of bank selling Super Tucanos to everyone fighting zombies.
- Open X-COM has timed missions now, seem very suitable here - say, rescue a scientist before a zombie horde from out of Czech Republic swarm his farm in the Alps. Or evacuate civilians to a safe point in X turns. Or disarm the bomb and kill the terrorists before their viral canister is ready for release.
- "UFOs" can be kept - as in, "Irregular Aircraft" piloted by criminals/terrorists/wealthy citizens/warlords. Maybe even civilian airliners and totally innocent craft, but you have to down them and kill everybody because there are infectees on board and its better to be safe than sorry.
- Governments as aircraft/missions to attack, with negative points as consequence but you can get some nice loot that way, PirateZ-style. Hey, their fault for not being more forthcoming with the things you need to defend them!
- Tech evolution would be interesting. I can see Z-COM early on using some military/police equipment they were given, but eventually reverting to more low-tech gear - bows, crossbows, ordinary shotguns and pistols, home-made guns, melee weapons, plate armor, etc as the ammo runs out. Does the game preserve partly-spent clips? I like the idea of going on a mission with a half-loaded assault rifle.
- I'm thinking tech would go widly differently from classic X-COM. Less Science Fictional, more Simple, Practical and Reliable/Early Future things.
- End-game weapons woud tend towards high rate of fire, big magazines, low firepower and Area of Effect. Stuff to kill large crownds of zombies - say, Man-portable Miniguns (Microguns is the correct term), Portable Cluster Bombs, Automatic Shotguns, Anti-Materiel Rifles for fighting human vehicles, mutants and enemies in Powered Armor, High-Caliber Marksman Rifles, etc. Good for killing numbers, but inferior to X-COM gear in portability/firepower.
- You can strip materials from things and use them to make other things. Crafting ftw! If one could implement equipment durability...
- Zombie captures are important as part of the investigation. Maybe giving extra damage as well.
- Different soldier types are possible now, right? If yes, I'm imagining you could separate random recruits in many categories, each with different stats:
- Refugee (average person that managed to survive, pretty much a normal dude with wear and tear taken off the streets and given a gun. All-around shit stats cannonfoddery)
- Trained Civilian (Civilians who are a bit more up to spec - More fit, with some sort of weapons or martial arts training, smarter, etc)
- Survivalist/Militiaman (survivors who got some limited training, good enough)
- Police/Conscript Recruits (Shoot straight most of the time, can take orders, doesn't break and run as easily, etc)
- Military (Good, decent soldiers. Nice and reliable)
- Spec-Ops (hardcore badass well-trained soldiers who can chew nails and spit napalm)
Of course, a shitty Refugee can eventually become as bad-ass as a Spec-Ops soldier, but he better have a lotta luck and be very badass to eventually level all that way. One cool idea might be that Military and Spec-Ops troops already have ranks, so you sorta have a small chance of randomly getting a veteran seargent or even find a army Commander among refugees.
About the End Game:
Spoiler:
If you want to use the aliens as being behind it, there could be a lot of nice twists, like say:
Twist 1: The aliens created the zombies to destroy human civilization, or failing that, so they could show up and say to all human nations "Hey pathetic homo sapiens, we have THE ONLY CURE - for the low, low price of... Planet Earth, and everything within."
Twist 2: Guess where it is.
Cydonia!
Twist 3: You don't know anything about no aliens or UFOs, other than what you find somewhere in the game - the fact they're out there causing this. Damn aliens!
Twist 4: You don't know anything about "Alien Alloys" or "Elerium". What's that?
Twist 5: That means no Avenger. So, how to get into Cydonia?
Twist 6: The old-school way, baby: A Rocket.
Twist 7: But not just any rocket - a BIG-ASS HUGE MUTHERFUCKIN' ROCKET that makes the Saturn V look like a firecracker. Because its not just going to Mars - its going to Mars, there AND back - to retrieve THE ONLY CURE! It needs radiation shielding, enough space and support systems for, say, 50 soldiers (you will need every one), enough weapons and armor to blow any hypothetical alien attack coming its way (or are you going to just build its smaller escorts?), etc.
Twist 8: You're not even building this on a X-COM budget, you're building this on a Z-COM budget. Its a freaking spec-ops home-made space program to freaking MARS made by a ramshackle organization that tries to survive against zombies.
Twist 9: You need to build/steal/research every component that huge rocket has. During a cataclysm situation. You need engineers, you need workshops, you need scientists and modern science. Also pretty much a entire base dedicated to launch it, make on-site components, AND a component to defend it.
Twist 10: Once you prepare it all nice and build it, then send your best soldiers to Cydonia, don't think that this is it already. Its not a Avenger, with the best tech we have I have heard estimations of 3 months to Mars with nuclear rockets. So you have to wait for three months until your rocket gets into Cydonia, and meanwhile the aliens go for broke and start frantically trying to destroy Z-COM and subvert/overwhelm with zombies all nations of the world that still exist.
Twist 11: You finally get to Cydonia! Which is... a lot like the final mission of the original X-COM. The aliens have all their horrible monsters and fancy plasma guns... except you don't. Your gear is made for fighting zombies/humans with good tech by then, not freaking aliens with plasma guns and psionic powers.
Twist 12: What is "psionics" and why are my troops shooting each other? You have no idea what is this, and no protection, nothing. You also never thought Plasma Weapons were practical. You don't even use lasers as side-arms - waste of precious energy.
Twist 13: Good luck!