If you're looking to write content that will eventually become part of the main game, here are a few guidelines to keep in mind:

All graphics:

• Use a Debian-compatible license. (Ideally, one of the licenses the existing artwork uses.)
• Provide an @2x version for high-dpi monitors.
• Provide access to all files used to create the graphics (Blender models, GIMP files, etc.) so someone else can edit them later if necessary.

Ships:

• Make sure the sprite is a single connected component so that collision detection works properly.
• Define sensible hardpoints.
• Balance the ship's capabilities with other ships in the same price range.
• 3D renders should be post-processed to look grungier and less artificial.
• Colored sections should span the range from red to yellow so faction-specific color swizzling will work properly.

Outfits:

• Provide outfit thumbnails at normal and @2x resolution.
• If the outfit is a secondary weapon, provide normal and @2x icons.
• Decide which outfitters will sell the outfit.
• An outfit's cost should reflect how good it is compared to other outfits of the same size.
• Where possible, have outfits be better in some ways and worse in others, compared to existing ones, rather than better in every single way.

Landscape photos:

• Should be photos or photo-realistic.
• Include both sky and ground in the image.
• Remove people from the image if possible.
• Photos should be realistic (no exaggerated color / contrast).
• Avoid foreground objects (trees, houses, people, etc.).

New star systems:

• The planet sprites with city lights should only be used for inhabited planets.
• Planet descriptions should roughly track whether they're in the hot or the cold range of the habitable zone.
• System name text should not overlap when the map is viewed at 100%.
• Try to put something funny or interesting in each planet description.
• Planet descriptions must be short enough to fit in the landing UI.
• New systems that are inside human space won't be accepted into the main game data unless there's a good plot reason for them.

New missions:

• Text should be as concise as possible. Remember this is a game, not a novel.
• The "yes, I want this mission" choice in a conversation should usually come first in the list (so it is the default).
• Conversations should not imply that the player is a certain gender or use gendered pronouns for the player (call them "kid" or "Captain" or <first> instead).
• Single-option choices with no follow-up text should be avoided, since adding a choice right before the [done] button turns what should be a single input to close the conversation into two.

New sounds:

• Use a Debian-compatible license.
• Provide a copy of your raw working files used to create the sound, if possible.