The hardest part of being a writer is the writing.
Writing is hard because it means pushing past the initial inertia and actually setting pen to paper (or fingers to keyboard, as it were).
An object at rest tends to stay at rest; a writer at rest tends to suddenly find the motivation for laundry, grocery shopping, lawn mowing, house cleaning - anything but writing. If this continues long enough, the writer is no longer a writer, but just a person who thinks about writing.
Writing is also hard because it means trying to extricate an idea from your brain space and transferring it into real space.
To borrow the words of a fellow narrative designer at ArenaNet:
"Ideas have no footprint."
An idea takes up zero space when they live in your head. They are amorphous and hazy, perfect in their ephemeral state, where they're just an imagined, shifting concept, where the parts you're focusing on seem sharp and clear and flawless, and the parts on your periphery are soft and blurred and Future You's problem to nail down.
The moment you have to concretely define the idea, craft it into words, pull it onto the page (or screen), is the instant it loses its perfection. It suddenly gains volume, becomes unwieldy, ceases to fit against the needs of whatever confines you're writing against. Suddenly you're limited by your own ability to craft a story, by the necessary structures of writing, by the logical flow and emphasis of what you're trying to say - and in the world of game design, also by word count requirements, length limitations, technological capacities, voiceover budget and talent availability challenges, implementation challenges, gameplay needs, and so on and so on.
It's great to be excited about your ideas. It's unwise to be too precious with them. Pull them half formed, ugly, and sloppy into the real world, plop them in the sand, let them roll around and get dirty and leave misshapen tracks that don't lead anywhere, a hideous affront to your attempts at wrangling words into some kind of coherent order.
Turn your ideas into writing, messy and raw. Give it a footprint; then cultivate and shape and mold it to stumble haltingly towards the direction you dreamed of in your head.