Can you boil down the overarching theme of your campaign into just one or two words?
And, why would you want to?
I have been thinking about this idea, and the essence of it is that if you know what your campaign is about at a conceptual level, it becomes easier to guarantee that each adventure sits within that theme, and you are not drifting toward something generic fantasy. It is also a useful prompt for when you are knocked into improvising. If you have to start making stuff up, if it is bang on the theme, it should be easier to reintegrate the stuff you made up on the fly with the stuff you have prepped for future sessions.
The theme of my Rolemaster Classic campaign is Deception. Dark Elves have deceived the dwarves into believing they were attacked by surface elves. The person the characters beleive is the villain is actually trying to prevent a brewing war between elves and dwarves. This person will use any means available and is one person against an entire conspiracy of dark elves.
Powerful artifacts that all sides want have been hidden in plain sight and protected by deceiving magics.
The list goes on. At first, the characters had no idea what was going on, but they have slowly started to pick at the loose threads and are getting a glimpse of what lies behind all the lies.
It is possible that in the next session they will work out that their early patron had been lying to them.
If my players flounder, such as when one of my ‘obviously brilliant’ clues goes right over their heads, I can easily make something up that will fit the overall theme of deception and misdirection.
If I need to get my characters back on track, I can have one of the opposing forces misdirect the characters back towards that groups natural opposition. The dwarves share evidence of elven treachery, the elves portray dwarven reprisals as unprovoked attacks, and the dark elves are just trying to drag both sides into conflict. It gives me enough levers to pull.
By sticking to my theme I am also signposting, bit by bit, what kind of powerful magics they are eventually going to have to face down. I am not going to go from deception, deception, deception to fireball.
I am finding having defined my theme that it is making my GM prep more fun. It is a challenge to weave the theme into, not every encounter, but all the big encounters and when building NPCs.