The best place I can think of to start answering the question is with the five elements that Aristotle said every story consisted of. This is nothing to do with the thirty-six dramatic situations or the seven fundamental plots or any of that how-to-write malarkey. These five elements are the fundamental building blocks of any story: Plot, Character, Setting, Theme and Style.
A story outline should cover all of those for your story.
You say you’re developing a story where some soldiers in World War One look out of the trenches and see that on the other side of the mud and fog and barbed wire is a shining otherworld.
That's a setting (at least half of one, the real world part) and the beginning of a plot. Plot is the bit that really conveys the high concept. High concept movies are strong on plot, soft concept are strong on character. A compelling plot premise is what makes people buy a book (Sleepyhead: a serial strangler isn't actually trying to murder his victims, he's deliberately paralysing them for life) or go to a movie (Flightplan: a woman wakes up on a plane to find her child missing, but everyone else denies she ever had a child with her).
Theme is what gives the story depth and a through-line the audience connects to. If plot is the reason you pick up the book, theme is the reason you take it to your heart and recommend it to friends. Theme also guides you in developing the plot beyond its basic premise. McKee recommends exploring the Theme, the Negation of the Theme, and what he calls "the Negation of the Negation".
So if the theme is Love, you'd contrast that with Hate (the opposite of love), Indifference (the negation of love), and Hate masquerading as Love (the negation of the negation). Those are all valid explorations of the core theme, each digging deeper and giving a more visceral twist as you find the antithesis that most strongly contrasts with your theme.
In your story, perhaps the theme is fantasy itself. Fantasy can ennoble mankind and make our solitary, poor, nasty, brutish and short lives worth living. I’m talking about fantasy in the broadest sense here – poetry, music, literature, art in general. Not goblins! In exploring that as your theme, you could contrast it with the negative side of fantasy: escapism. Escapism just says things are crap so let's pretend they're not real. The "Negation of the Negation" is when we take our bad experiences and impulses into the realm of imagination and end up sullying it with them. In your story that could be by the characters bringing disease and war into the pure otherworld they’ve found.
Now you can populate the story with characters. Who are these guys sheltering from an artillery barrage? Let's say you've got a door-to-door notepaper salesman, a communist automobile worker, a conscientious objector medic, and a chorister who got into the army by lying about his age. The story will take each of them on a journey that has some resonance. The notepaper salesman has made a living out of selling the raw material on which a million people write their dreams and hopes, for example. Sometimes the plot and the theme suggest characters, and those characters change your first idea of the plot, and so on.
That gives me some idea where you might take the plot. Say that the fantasy world is a utopia, but your protagonists find only children living there. Later it turns out these are all the millions of children who will remain unborn because their fathers were killed in the trenches. So they are literally the hopes and dreams of those men that live on despite the existence of suffering and death.
Obviously to "squeeze the lemon" you will then have to have one of the characters meet the son he is destined never to have. He refuses to go back and die in the trenches, but that causes his son in the fantasy world to grow sick.
However, I wouldn't expect the story outline to set in stone how the plot was going to develop. That's something most writers would want to allow to develop organically. The outline should set out the plot premise ("a man is chained to the wall and the hacksaw next to him isn't sharp enough to cut through the chain") and only needs to sketch out where you would then go with the plot.
Once you've sketched out the theme and direction of the plot, you start to see where the style might borrow from. In the case of this storyline, I'd look at the war poets, and also at the Utopians like H G Wells and what they wrote after WW1 about building a better world. Why not style the otherworld on that? (A bit like Priestley’s imagined post-WW2 utopia.) Those are ideals that are still around today and would give the reader or viewer something to connect to.
The synopsis should be a few pages outlining who the characters are, where they are, what happens to them, why and what is the style/tone. Some people like to begin with a guide like this one:
- Once upon a time...
- And every day...
- Until one day...
- And because of this...
- And because of this...
- Until finally...
- And ever since that day...
- Bill, Fred and Jim were in the trenches
- And every day the guns pounded
- Until one day the gas drove them out
- And they ran into a magical utopia
- And they brought war to the utopia
- Until Bill saw that to save the purity of the place he'd have to go back and die in the war.
- And over the years, as Fred and Jim grew old, they could still see that utopia just across the fields at dusk.
I'll just add one caveat. A good story outline is a nice thing to have. It really does help you make a sale to the studio, network, or publisher. But in striving for the very best story, you can get distracted. Like, you know that steak they sell in supermarkets? It's bright red-orange in colour but that isn't what the best steak looks like. The best steak is hung for a week or three until it goes dark brown. Supermarkets dye their meat orange because that's what people think fresh meat ought to look like. And in the same way, having a good story isn't the most important thing for reaching an audience. They only get to see the story once they’ve bought their tickets. To grab their attention in the first place you need a hook. Fantasy and the Great War both seem at first glance to be a very long way from our lives today. Yet you have to make people stop and think, “That’s a story about me.”
But we’re talking about your story, not mine. And all this is free advice. So I’ll leave it there.