Okay, I'll play too. For a minimum viable "ship interiors" product, I think you'd need:
* Cockpit
Yes, that's it. It's the obvious low-hanging fruit since they're already partially built. There's not a ton of gameplay potential, but I like the idea of say being able to walk to the rear door and choose different egress points on ships that clearly have multiple options. This would have meaningful tactical gameplay impact - e.g. popping out of one of the top hatches on a 'Conda to start with high ground if landed near hostile ground forces, or deploying multicrew team members from the cargo bay in the style of CZ drop ships.
I would note that station interiors establish that the ED universe has Trek-style anywhere-to-anywhere turbolifts, so logically that's what should be behind the cockpit door on most ships. Something like a Sidewinder or Eagle is small enough that it might seem kind of silly to have such a system installed (and the interior modelers would have a field day working out where to put the turbolift shafts), but if they were going to do it at all I rather imagine they'd prefer to treat all the ships consistently.
Also -
I like that idea a lot! A cabin, as others have noted, is an obvious Arx-powered customization point, and respawning in an emergency medbay cot would add at least a tenuous gameplay hook to the space.