Running a One-Shot, and overall Campaign Plot Help

[doc neon]

Hello Playground!

Long time lurker, first time poster, et cetera. I hate for my first post to be one asking for help, but I seem to have stuck myself into a corner, plot-wise, and also need a one-shot adventure, for a session... tomorrow.

I suppose I'll start with the latter. In the first session (incidentally, my first session DMing, and most of the player's first time playing) of the campaign, the party cleared out a cave of monstrous spiders and assorted baddies to help out a small poor village. Standard stuff. Now, though, it seems that a good number of players won't be able to make this next session, but I'm still getting asked to run something. I don't want to cut anyone out of the plot, so I'm thinking of rolling up some NPC-class characters from the village that was helped and having the players take them through an adventure. The only problem is that I don't know what adventure it would be. Trouble with the corrupt government/a gang/other villages? Dealing with the aftermath of the players clearing the cave? Trying to get more supplies? I'd like to stay away from any kind of conventional dungeon crawling, since the players could do that on their own, and maybe let this serve as an introduction to role-playing, as opposed to just rollplaying, which we've been doing up to this point. Any suggestions?

In addition, I've currently got something of a "Cold War" situation planned out for the players. Since we're not far into the campaign, I only have a very rough sketch of the rough outline that the players will, presumably, follow, or at least get a series of enticing hooks towards.
1. There is a cold war between two superpowers. They are both neither good nor evil.
2. There is a secret conspiracy to escalate the war, so that a third party can come in and conquer both weakened nations.
3. If the conspiracy is revealed, the third party will invade regardless, and the two superpowers will hopefully unite against it.
The only problem is that I don't know what the third party is. I presented the campaign setting as the two superpowers being just about the most notable countries around, and I'm not sure where I'd get a nation powerful enough to challenge both of them. I was tossing around the idea of using demons, outsiders, or undead, but I want a feeling of epic, LotR-esque warfare in the war. Of course, I could say, "Here's a bunch of warriors that no one noticed, now fight them," but that violates the established setting a bit. Alternatively, I could just quietly start playing down the "superpower" status of the countries in the Cold War and have NPCs talk about foreign nations more, but I feel that the players might catch on. Any thoughts?

[JustPlayItLoud]

What about a mixed force? Say, skeletons that have been raised en masse from rural graveyards that tirelessly attack places of minimal strategic value, foreign mercenaries that make up the standard ranks, and perhaps some sort of special police/knightly order that has been working for the villain all along to strike at valuable targets from the inside.

All this would require is a wealthy necromancer and a greedy police/knight captain. One of them would preferably be a good strategist. Perhaps there could also be orcs, goblinoids, or other savage/evil humanoids that have been hired or even pressed into service as slave warriors.