well, this sucks, i cant model the frakking disk thing on the wanderers ship.
psh...my ideas suck.
You can't just hit "Create PolySphere", delete half of it, and then cap the hole? Seems fairly easy enough to me (assuming I'm thinking of the same thing you're having trouble with).
Here's the thing, yes there are loads of ships not in TBP, but what ships are they? Seems to me, that most of the ones that were left out were seen in maybe an episode or two and never heard from again.
I know that there a few ships from Crusade that would be fairly easy to model. The Technomage ship is essentially a wedge with a tail fin, all black.
Most of the First Ones ships are pretty basic in terms of shape and design, just need a good texture job. Soul Hunter vessels again are pretty basic. The only things I can think of that would be a challenge would be the Liandra, Hand vessels (due to there being no good reference images on the net of the thing, just really small promo screen grabs from Sci-Fi.com), Enfalli, and maybe the Thirdspace ships.
Heck, with decent refs, modeling the Hand ship should be pretty easy as it's symetrical (essentially a dark snowflake).
Really though, most of the ships I mentioned would only be needed if there were missions centered around them. The First Ones seldom appeared and were only used in combat during the Battle of Corianna 6. Liandra I believe is one of a kind patrol cutter, the Enfalli, not sure about. The Hand were seen once and never referenced again (seriously, I wonder what thier bag was), Soul Hunters again, while existing are very rare. Thirdspace aliens only appeared in Thirdspace.
The Streibs ship you have there only appeared once and they were rumored to only come out once in a blue moon (and were pretty weak).
Again, if you were to go for new ships, I'd focus on going for new League vessels like the Abbai Gunship that the Into the Fire space sim had concept sketches are even take a crack at developing the other Shadow Allies' vessels (there was another race or group not affiliated with the Drakh that were going to be in the game, but I can't remember who).
But asides from that, try making a campaign that uses the ships that are already in TBP unless your story absolutely calls for a new craft not in there. Then just learn how to model and texture (follow through a few tuts and you should be good).