The TOS interior is great, dont get me wrong, but you've opened a can of worms here by finishing it - you've shown that cryptic can make better interiors, with more interaction and proper scaling. The three interior layouts we've been using in the past just wont cut it anymore.
I figure you guys will have a lot on your hands, designing and releasing various interiors in the future, but I'm gonna jump the gun here and make the first request: Let's get an NX-01 style bridge and interior pack. Personally, I've always liked the more utilitarian and militarized look that enterprise had, it wasnt all 'shiny and clean' like many of its successors and it looked more like the inside of a nuclear submarine - ready for battle - than the inside of your grandmother's minivan (no offense to the TNG guys but... the tan was a little sickening).
There's also the allure of more conventional technology - actual showers that use water, a less advanced sickbay, military bunker-style housing for the majority of the crew, so on and so forth. Below I've got some images linked, and descriptions of specific areas I think should be included with the interior...
The enterprise bridge is a little more compact than most of the bridges we have currently, though it's pretty similar in scale to the new TOS bridge - though with more console detail. The MSD is in the back of the bridge, in the rectangular extension to the room; I cant quite remember but I think the door at the back of that section goes to the ready room/briefing room.
The overall look of the bridge is quite dark and very military styled - rather than using the comfortable 'curves' most of the bridges we're used to, everything is relatively square or angled.
Even though the captain is the 'big dog' on the ship, the NX-01's design kept true to modern day military styling... even the captain doesnt get a ton of room. It's a bit ridiculous, to me, to go on our current interiors and see a captain's room that could easily double for an entire crew lounge and then some, simply too big for one person. The captain's quarters on the nx01 are relatively small and to-the-point, no extra comforts, just a bed, a desk, and a shower (and the shower must be included, after all it was quite stressed in the show that it actually uses water instead of sonic waves like the TNG+ showers).
The Captain's mess/dining room should also be included, with a small section on the wall for trophies.
The nx-01's corridors, on the crew decks/upper decks at least, were quite small, even compared to the TOS interior we've got now. There really isnt a lot of room to spare on a starship, so no sense wasting it on open hallways. The corridors on those upper decks, as you can see in the images, are built square (much like the TOS interior) but the bulkheads form a rounded/tunnel like visual effect.
The enterprise sickbay was the only 'bright' area on the ship, and even then it was still relatively dim-lit compared to TOS/TNG era ships or the interiors we have available. The biggest thing to note is that the enterprise sickbay uses more 'modern' style medical equipment, rather than the high tech futuristic stuff we're used to - make note of the MRI-style scanner, for example.
The sickbay should also include the small decontamination chamber used by tucker and t'pol in season 1, blue lit for sterility.
A relatively small, low-ceilinged area with a proper kitchen/bar and a handful of small viewports, as shown in the picture. There were no replicators on the enterprise, at least at first, so including the kitchen area is a must. Would also be nice if all of the chairs worked, as on the new TOS interior.
On the lower/engineering decks the corridors are better lit, show more bare steel, and are wider than the standard corridors - most likely for moving cargo and other bulk items through the bowels of the ship.
The armory, or torpedo room, at the front of the lower decks - with the two notable torpedo loading tubes and the torpedo storage racks off to the sides. While none of the current interiors have a similar room, I think it should definitely be put in the nx-01 interior, it was a key part of the ship in the show. Could replace the engineering lab/office, since I dont believe the nx01 had one.
Another room that really sets the interior apart from current designs, the Nx01 had a horizontal warp core, rather than a vertical core (much like the klingon interiors we have in the game already, which use a horizontal core - albeit a much, much larger one). Personally I always thought the horizontal core looked better and made more sense - no working against gravity and easier to work on/maintain.
Since we dont actually have the full interiors or access to all decks in the game, I'd split it up like so: