I had a thought about a zombie outbreak mod while in the shower yesterday. Unfortunately it just wouldn't mesh well, I mean what are your tanks going to oppose, zombie infantry?
You might be underestimating the zombie as a creature. Its already dead, so it would be very hard to kill off a Zombie unit. Sure, you can smash every single living corpse into smithereens, but your own dead soldiers will be rising up to fill their ranks. So, they would "heal" really fast and require one last hit before being totally disintegrated, making them hard to finish off.
Furthermore, zombies aren't about individual strength, but rather the numbers. A zombie outbreak would start in a city, causing either a small "incident" (maybe loose a population point and damage some units) - or a total collapse of society. If the latter were to occur in a game of Civilization, the city would be razed and every population point turned into a number of Zombie units.
The aim would, then, be to isolate the infected area on the map. The zombie epidemic would travel both via contact with the Zombie units (which will disperse in every direction) - and via trade routes (as does the Plagues of RFC). The nearer the Zombie unit is to a city, the greater the risk of another outbreak in a metropolitan area (which is the catastrophic thing you wanna avoid happening). New Zombie units would arise from any improved tile that the existing ones would traverse. (Also, the Improvement itself would be pillaged. This makes Towns, Villages and Hamlets extra precarious, since they will spawn more than one Zombie unit!)
Your units (at least your modern ones) would be able to smash the Zombie units quite easily, but as mentioned they're not easy to kill of completely and if they are not dealt with, they'll just heal (collect more dead people in their ranks). The point is, of course, that there will just be more and more of them, while your own resources in terms of cities and improved land diminishes over time.
And when you think you've got them all, every razed city is still a source of disease and could infect new cities any time. Every inflicted tile could be covered in a Fallout like improvement requiring scrubbing. This, of course, is risky business and the Worker doing the scrubbing would be at risk of causing another outbreak (turn into a Zombie unit).
You would only be able to call the entire thing off when every single Zombie unit has been destroyed and every single infected tile been scrubbed. Until then, it can and will just start all over again, sooner or later.
One thing to consider is also your neighbors. Firstly they'd call off any and all Open Borders, crippling your economy.(Otherwise, the disease could spread to them, too.) Secondly, they could take advantage of your situation and opt to invade. This is of course foolish of them, but not very unrealistic either if they are your arch enemy or you're occupying some of their turf. So, a manageable situation could easily turn into a planet wide emergency if the international community isn't pulling together at the moment of crisis...
I like your ideas for the alien invasion mod with some exceptions...
Well, you can't please everyone and those were just some ideas of mine. I'm sure an Alien invasion mod would be very different from my own vision, since I'm not doing one myself.
Your own ideas I do like however, even if they would call for a somewhat predictable scenario. I do second your ideas about natural resources, though, and it could be a key objective for my Resistance Fighter units to pillage/sabotage these. When I mentioned new Technologies I wasn't thinking about Alien Technology as such (even if the Aliens would have there own Techs) but rather inventing new ways of combating them. Like Alien Biology, allowing you to identify the invader's weak spots. This in turn could allow a new unit, promotion or building with the aim of defeating the Aliens. Another Tech would be Alien Language, giving you the option of gathering intelligence on them (maybe direct your espionage efforts on them).
Regarding rival leaders of the world, the common enemy would automatically give bonuses to all relations, especially over time. I don't, however, think that it should be unthinkable for the Earthlings to ensue their petty disputes even in the face of an Alien invasion. Everyone wants to be on top when the dust settles, after all. So no Permanent Alliance between all original Earth civs, please!
The best thing would be to have a merged mod with an Alien Invasion, Zombie Apocalypse and maybe some other global calamity. As you said, it could be entirely random if and when these occur in a single game. The Zombie epidemic could however spring up again and again over the course of a single game - or just be a single incident - with the potential to become a global one. The thing would, however, be that you'll never know! So you'd be playing the game as usual, preparing for just about anything and everything...
