Minimal viable implementation.
Just an elevator. Nope, not really interiors, just an elevator.
Just a room for cosmetics. To have value, cosmetics need to be a part of the natural gameplay flow. For example, paintjobs are visible to others and for screenshots/videos with external camera, both happen while in natural flow of the gameplay. Dashboard livery, although not visible to others, is always visible to the player and in videos/screenshots, also in natural flow of the gameplay. The room with the only function of storing cosmetics, and the only gameplay of load in to look at them and load out to play the game - won't even give any value to such cosmetics, simply because they won't be seen in the natural flow of the gameplay.
I see no point in interiors that have no other function except a place to put cosmetics in completely out of everything else in the game.
At minimum, interiors need:
1. Ability to leave the ship going through interior, and get back into a set going through interior. Doesn't matter if there would be a teleport option or not, but without this basic function - there will be not much use for interiors and almost no immersion from them.
2. Interiors need to have something interactable. If it's just a corridor with doors, yeah something, but not even a good space to put cosmetics in.
Maybe some extra terminals with additional info (exploration/exo-bio logs), maybe an equipment rack to switch loadouts or weapons in current loadout without a manager menu, maybe a direct access to fighter/SRV to get out in it without a menu. Basic function and immersion at minimum. For that interiors need to have rooms, or at least one as big as reasonably possible for a specific ship.
This ^ 1 & 2 is the very basic implementation that could give the immersion of playing as a commander in a ship (and not just a ship), and that is what would give cosmetics their value, because the whole thing will be an intersection for the gameplay/immersion of the rest of the game.
Although it'll already be HUGE for me and I'm sure for many players, I don't think it's enough for a wider audience - because it doesn't add much in new gameplay and in new ways to interact with the game world.
- To have a possibility of repair minigames there is a need to have these modules accessible, otherwise it would be the same cockpit menus from another terminal. So there is a need for at least a room with PP.
- To have a possibility of other minigames (labs, workbench, etc) there is a need to have space for that too.
3. Creating all the models and tech for interiors is a lot of design, art and development. To limit all that to only players ships doesn't seem like a good idea. It's only makes sense to use them for everything they can give to the game - NPC ships, wrecks, abandoned, disabled, ... for missions, for signal sources, for exploration/lore bits, etc. That is where the vast majority of possible gameplay interiors can offer could happen.
If it's only for ships/wrecks on the surface of the planets - at least something, but very limiting, as we already have planet content, and ship interiors is much more about space and not planets again. Planets need other content/atmospherics/caves/storms/etc, not ship interiors. That's why there is a need to have mechanics to get to these other ships/objects in space, either boarding limpets, or EVA.
We already have ships flying, on-foot planetary gameplay. The only interaction with the game world Elite is missing - is being a commander in space, and not just a ship in space.
Being a person in space is, in my opinion, the main possible attraction of interiors for a as wide of an audience as possible.
4. Gameplay.
For player' own ship, the minimum of - a good interior with interactable objects and more immersive/natural access to vehicles, an ability to go through an interiors to get out and in, some repair/other minigames, cosmetics. At minimum.
For other ships / object that can use ship interiors models/textures - there needs to be something to do. This way there will be something to actually play the game for a wide variety of players, and it could also help with cosmetics, as after all the same DeLacy interiors players could be much more inclined to make their DeLacy interiors unique.
This way, ship interiors will accommodate a wider audience of players who might be interested in them = more players overall in the game with everything good that leads too.
imo, If Ship Interiors would ever happen - they will have both gameplay and immersion, because without them it doesn't make much sense to put all the effort/time/money into creating models and developing tech.
It doesn't have to be all possible gameplay ideas from the start. it doesn't have to be all of the ship's interiors from the start. Doing them by manufacturer and selling Early Access / mini-DLC is a very good approach. Each such addition can have Interiors of one manufacturer and some new gameplay loops for all interiors related content.