LOST SHIPS! While Wing Commander 3 is chock full of completed-but-unused assets, Wing Commander 4 has very few. And here they are! First up, this Black Lance carrier is pretty sleek:
Here's a Confederation carrier under construction; it's NOT the TCS Princeton (CV-48) which appears in the Speradon series so it's not clear at all where this would've been used.
And the last one I've found so far, a unique Border Worlds starbase.
