"Now hang on a minute. How many ships do you have to work with?"
It's not the numbers, it's the different races. The races currently in, are the well-known races we've come to know and love through the show.
With only well-known ships in, there is only so much possibility for surprises, new encounters, mystic derelicts, etc.
"Now hang on a minute (again). That's just the plot of Thirdspace isn't it? Seriously though. In my opinion everyone has enough ships now to produce a decent campaign if they want to. All that is needed is imagination, positivity, and flexibility."
Yep, an example of that is the Thirdspace plot. Which was really interesting, I think. Also, considering Lyta mentioned that the mistake was "one of many", there is

of room to work with the Vorlons. Also because we really don't know anything about them, and their history, except the shadow wars every thousand years.
We know the human history, we know recent and some old Minbari history, we know recent Narn and Centauri history.
Those are mostly the ships currently in(Not saying there are few, but just a limited selection, since we know so much about the owner races.). The Drahk raider ships was a nice addition, but it still lacks the Carrier, to make it really useable.
Now, the point wasn't really to argue about this, but just let you have an idea of what I thought was missing. Fighters, perhaps bombers, and a couple of capital ships, from races we know little to nothing about, would make all the difference.