I don't doubt that you would have "blank page syndrome" after your prodigious outpouring of creativity recently. Hans Holbein, and massive city-sized starships! Maybe what you're experiencing is akin to post-Partum Depression, after delivering all those children of the mind. ;-) 
Since most of the spaceships you design look like human-type high technology, perhaps you could try a more organic style spaceship design; something with flowing lines, something that looks like it may have been grown rather than manufactured. Imagine a foe for the races that would use the ships you've already designed. Good luck! 

Edmund