Thirty Plots for Seven Samurai

The Seven Samurai plot has been used in everything from A Bug's Life to Mass Effect (and every other Bioware game) to Red Hand of Doom but damned if it doesn't still work. The enemy is out there, with numbers and power too great to confront directly, and they're coming to wreck your shit if you don't find some seriously impressive way to even the odds fast. It's such an archetypal plot but you can fight off the horde in so many ways and it leaves so much room for improvisation that I never get sick of it.

Prepping for an adventure like that you need to know who the enemy is and what they can do, have a rough timeline for when and how things go bad without opposition, and think of a few likely ways the players can help their allies or hinder the enemy. But if they try something else you've already got the framework of how the enemy reacts to being foiled, maybe some cute little point chart for how minor victories and defeats build over time, and there's room for both unorthodox solutions and different degrees of partial success. Like if in your prep work you decide that blowing up the bridge in front of the enemy's march delays an attack by two days, but the druid convinces the mammoth king to stampede the enemy's baggage train instead, you already sort of have a plan for that even if you didn't know your setting even had a mammoth king until the druid's player suggested it. It's all the freedom of a sandbox setting but with the urgency of a doomsday clock - they can attack from any angle and retreat when they need to, but the army's still marching and it won't stop itself.

If your system of choice has some sort of city-building or dominion rules system (or you're feeling ambitious enough to make one), nothing makes players more invested in a location than having it be one they've built and managed themselves. It's the old "give them the sun and make them fight for the moon" bit, but again, somehow it always works.

Of course the downside of being able to try anything is having an infinite number of things to try - uninformed decisions are basically just coin flips, so scouting out priorities and opportunities should be quick and painless. If the war effort is vast and your players are in positions of authority, it's nice having a friendly advisor NPC around to handle routine scouting reports and suggest "Well if you don't have any business of your own to attend to, we've learned that these locations are particularly on fire today..."

THIRTY THINGS YOU CAN DO TO HELP THE WAR EFFORT

Many of these were inspired by the Myth series, which doesn't get enough love

1. Protect or recover a gold shipment sent out to mercenaries
2. Capture and train beasts of war, or liberate beasts forced to fight in a gladiatorial arena
3. Convince reluctant or blackmailed allies of the enemy to switch sides
4. Stir up internal dissent and incite rebellion
5. Rescue prisoners of war or smuggle out an enemy that wants to defect
6. Defend an isolated town from an imminent invasion
7. Clear a ruin or stretch of wilderness of monsters so you can build or repair fortifications
8. Sneak behind enemy lines to sabotage a factory or raid supply lines
9. Blow up a bridge or train tracks, cutting off enemy retreat or securing an ally's escape
10. Steal enemy battle plans or ciphers, or disrupt communications
11. Hunt down and kill or kidnap key enemy personnel
12. Root out a scout, spy, or assassin in friendly territory
13. Hold off against overwhelming odds until reinforcements arrive or a task is completed
14. Destroy or steal stockpiled ammunition or weapon prototypes
15. Kill or capture a traitor before s/he can give away plans to the enemy
16. Defend a foreign diplomat, merchant, or scientist from assassination
17. Harass an army to draw off soldiers before a major battle
18. Scuttle a flagship or moving fortress
19. Uncover the secret weakness of an invincible enemy
20. Gather a crucial ingredient to build a magical device
21. Help a town recover from a plague or natural disaster
22. Destroy whatever means the enemy is using to bring in troops behind your defenses
23. Negotiate an alliance with a neutral nation or strange outsider community
24. Recover an artifact from an ancient ruin before enemies can do the same
25. Set up an ambush for a group of troops separated from the main army
26. Secure a route through inhospitable terrain to attack from an unexpected direction
27. Take out an artillery battery or fortress to secure a key location
28. Curry favor with an organized crime syndicate to smuggle in needed supplies
29. Consult an oracle or sage at a hidden temple or place of power
30. Stage an elaborate heist from a bank, train, airship, casino, or treasure galleon