I have been privy to a number of plot conversations, where the plot team said they want to do something epic that everyone can take part in, and that everyone remembers for years to come. The problem is that a lot of times, plot teams are in the tabletop mindset, where you only have to involve 5 or more people, and where one plot mechanic is often overlooked.

Start a full-fledged war.

Every few years, dropping something like an all-out war on the players makes for an incredibly immersive and entertaining game for the players. Here are some of the things you'll be able to take advantage of when you start a war.

Streamlined Plot
A lot of times, plot gets hung up in the details. Players often focus on the actions they need to perform and lose sight of why they're doing the action. One way to avoid having players not see the forest for the trees is to avoid showing them the trees altogether! Instead of giving the players actions to fulfill, make them aware that you're at war, and they'll tell you what they what to do in order to fight that war. The plot writes itself!

Heavy Combat Base
War plot is based around a heavy combat base. Now, I'm not saying that combat is more important than roleplaying or problem solving for your game. What I am saying is that it's often easier and more cohesive integrate puzzles and politics into a combat module than it is to inject combat into roleplaying modules. You can normally use puzzles as mechanics for IG acts, but you can't use combat as a simulation. Taking the combat base of the war and adding the other aspects of your game to it gives you an easy way to please all of your players.

But please remember, the elements of problem solving and roleplaying are still a huge part of the game, so don't ignore them!