If you're designing theme-first, you must choose a fascinating theme. When you tell people the theme, they should immediately be interested.
Players and publishers are not interested in "a pirate game" or "a train game". To a jaded publisher, a game about the sewage system is vastly more interesting than another game about zombies. I kid you not. Your theme should be interesting, but doesn't need to be totally zany. Zany is definitely good though, if you can manage it. If a publisher somehow gets around to evaluating a boringly-themed game, and the mechanics interest them, they'll just retheme the game to something else — of their choosing.
My game Radlands was originally a generic futuristic game, but the publisher changed that to an over-the-top Mad Max theme. It was a huge improvement.
Don't make your game literally ten times harder to publish, by choosing a boring theme. It's ludicrous to have complete freedom with the theme, because you're designing theme-first, yet not actually choose an interesting or novel theme.
Even if your theme has been done before, and you don't want to change it, you can still put a new twist on it. If you have to do zombies (which you shouldn't), make it "zombies in Japan" or "zombie cats" or something.
I'm not really wanting to just evoke the theme itself. The theme leads to a larger feeling of the theme.
I don't just want a pretty, pirate-themed game. I want to feel like a pirate — roaming the sea, attacking other ships at will.
I don't want a spy-themed game. I want to feel like a spy — planning secret moves, and having secret information.
In case this distinction isn't clear, think about some examples from real life for a moment, without thinking about board games.
Imagine you're a kid arriving at a theme park, and there are more rides and activities than you can imagine.
Imagine the feeling of exploring a haunted house.
Imagine you're at the edge of a vast, unexplored wilderness.
Think of a game you really like. What is the feeling you get when you play it, or that you got when you first played it?
Turning a real-life feeling into an actual game is extraordinarily hard, but you only need a small amount of that feeling to come through in your game, for it to make an amazing board game. This is done not by slapping a theme on your game rules, but by carefully choosing game rules that bring out your theme.
The size of the theme should also match the size of the game. Do not try to attach a big story to a small game.
In a friend's prototype, you place object tiles around
cat cards. When a cat is surrounded by objects, you add up the points on
the tiles, to see who takes the cat. This game was mechanics-first,
other than the very general "cat" theme.
We went through all kinds of big-picture themes for the game. It could be a
cat adoption place. But why would you adopt multiple cats, by placing objects
around them? I suggested a cat cafe. That was better. However, nothing else in
the game said "cafe". We agreed in the end that the story was just that you were
having fun with cats. You give them objects, and they like you, and come to you.
That was the extent of the story. It was the correct choice, because it matches
what the game actually is.
I've seen numerous basic number or colour-matching games, with some kind of epic legend as a theme. Don't just pick the most awesome theme you can think of. Find whatever matches your game best.
Your theme should be flexible, so leave it broad at first, and narrow it down as you build the game. You probably won't retheme the game, but let it move around in scope, setting, and time, as its components develop.
You need to be able to explain your theme in a few words. Telling a long story you invented, is not a theme. It's just a random collection of things.
I'd like to play a game about running an ant colony, or creating a mosaic. I know what those things are, and I want to play them in a game. I'm not so interested in being the Archmage of Kronglor, or recovering the Five Galactic Shards of Kozmog.
People need to be captivated by the theme of your game, and that's only going to happen if they can actually understand it.
I've designed games on all kinds of different themes, and people's reaction was usually fairly muted. Then, I started making a game about kids roaming around a neighbourhood on their skateboards, and getting up to all the mischief that kids get up to. This theme resonated strongly with people, and they were immediately fascinated and engaged by it. If you're designing theme-first, this is what you should be aiming for.
Having said all this, I don't recommend starting theme-first.