- really big -- the number of people who would pay $100 for a non-Enterprise is questionable, and that's probably where the price would start for a Warbird or Borg cube in any preferred scale.
Why does the Romulan Warbird have to be huge? Just because it was huge in universe, doesn't mean a toy would have to be. The Playmates Romulan Warbird was 14.5" long by 9.5" wide, and I'd say its a pretty good size for that ship. Are you saying a ship that size would cost $100?
You made the Klingon Bird of Prey absolutely huge, it shouldn't even be remotely that size compared to other ships that you've made, so scale is completely irrelevant when it comes to your ships.
- yet another variation on a Klingon vessel -- we just made arguably the most famous Klingon ship class, so I can't imagine we'll revisit that well soon, especially since the TNG ships are bigger and, frankly, clunkier
The BoP certainly is famous, but it is hard to argue whether it or the D7/K'Tinga is more so. You probably would have gotten more variants out of it than you will out of the Romulan Bird of Prey.
Also, you really think Trek fans would have complained about getting two Klingon ships in a row, especially if it was the D7? I'm sure there would have been some, but there probably would be more excitement for it than there is for the RBoP. (That's not to say there isn't excitement for it, there certainly is)
- an obscure Federation ship -- no one class really stands out, and most are based on a similar model to the Enterprise, like the Reliant is
The Prometheus is the first ship that comes to mind, it's fairly popular and doesn't share any design similarities to an Enterprise. After that I'd say the Akira, Norway, Sabre, and Steamrunner. Would these ships sell enough to justify being made? Probably not yet.
- an obscure ship from a minor race -- how often did we see the ships of the Vulcans, Ferengi, Andorians?
Enterprise featured ships from the Vulcans pretty regularly, and I'm sure I'm not alone when I say they're some of the coolest alien ship designs in Star Trek, I'd kill for a D'Kyr type Vulcan ship. The Andorian Cruiser was also featured quite a bit.
Lets not forget that the Romulan Bird of Prey in Enterprise is a hell of a lot nicer looking than the one in TOS.
- the Enterprise C -- it's historically important, but it appeared in one episode
The Enterprise B had arguably the same amount of screen time, if not less. Thank god you managed to make the normal Excelsior out of it, or we probably would never have gotten it either.
The original series is the essence. TWOK is just the best filmed representation of said essence.
TOS started Star Trek on its journey, but TNG and onward defined what Star Trek is now.