I would like to see Armageddon have a bigger and more lasting effect then it has at present. The current form honestly feels like a bit of a letdown when it finally triggers, even though the death it causes in unparalleled.
What follows is a quick list of random things I've come up with that would make Armageddon more fun. Feel free to pick and choose which one's you like.
More Hellgates
In addition to its other effects, Armageddon now also spawns another batch of demon spawning portals all over the world. For extra fun, give the spawned units stigmata and up the spawn rate from the original portals. Also be nice if they would turn the tile they spawn on into hell terrain.
Demon Barbarians
In addition to the usual orcs, demons of varying power are able to spawn in the world. Imagine the survivors' dismay when Balors start to pour in from the wastes.
Destroy Cities
Either a random number of cities below a certain size or just a raondom number of cities would work. It feels downright strange to have the worst thing ever happen to the world but still have all my bastions of peace and tranquility relatively intact. You could set it to destroy all cities below a certain population, but I think that would be a tad harsh on our Kuriotate friends.
Hell Terrain Everywhere
Cover the world in hell terrain when Armageddon hits, leaving only the cultural areas of the Good untouched. If there was a way to randomly create large patches of it hither and thither, that would be more aesthetically pleasing. At the very least, double the spread rate of the current hell terrain. Would work great with the idea of the demon spawning portals creating hell terrain at their location.
Cultural destruction
Reset/lower the culture of all cities and slow its growth for a number of turns. Erm...might be a tad harsh on those pursuing a cultural victory...but perhaps that's a good thing. Right now, the only thing you can do is wage a war which may or may not be feasible given your situation.
Anyway, getting rid of some culture has some nice effect to it. First, it nicely illustrates the decent of man's position in Erebus. Everything has gone to crap; no one has time for arts and crafts anymore. The need for survival has taken over, if briefly. Secondly, it gives more areas for barbarians to spawn. Nice synergy with the demonic barbarian ideas. Thirdly, it would allow hell terrain to spread into areas once owned by good Civs, meaning they would have to spend some time pushing it back once their culture is restored. Nice illustration of the new world struggling to cleanse itself.
AV Civs Gain Barbarian
When Armageddon hits, all Civs following AV gain a copy of the barbarian trait, save that it will never go away due to culture. Instead, it goes away if the Civ ever converts to another religion. Later converts to team evil do not gain the trait of course. That train's already left the station.
There are a couple nice synergies with this. First, the evil guys gain some benefit for being evil. Would also help to further polarize the world as the player must make the decision before Armageddon hits whether to embrace it, allowing hell terrain to spread but gaining a useful trait. Also means the few remaining good guys and neutrals will have the bulk of the barbarian troubles.
Horsemen Rituals
Ever wanted to command the unholy quad? Now you can! Just trigger Armageddon and complete the vile rituals and they will bend to your will. Did the horsemen already get whacked?
No problem! These handy dandy rituals will raise the gods' nightmares back from whatever pit the were buried in, ready to wreak havoc once again. Be warned though! Just because you control them now doesn't some other Civ can't do the ritual themselves, gaining control of them no matter where they are. Don't worry though, as you can repeat this ritual as often as you need.
Goody two shoes and neutral Civs need not apply. May require certain techs and the horsemen may be slightly crazed.
Create Your Own!
I hope you like my ideas. Especially any of the devs who read this thread...
Anyways, feel free to comment on mine and come up with some of your own. Let's turn Erebus into the diseased wasteland of pestilence and death it ought to be after the fires of hell have ripped it assunder.