I always liked the concept of FASA's L7 "Bringer of the Nether World" fast strike cruiser, which was an obvious rip off of the Federation's fast cruiser design by nachelle switching programme that date back into early TOS. I've modelled all of these Federation test bed ships from the various diagrams that exist. All early ones used the round PB-38 and PB-40 type nachelles. Their was also one nachelle switching frigate, the Rapide, which was based on a Burke class frigate saucer. I modelled that prototype in October 2011.
The Feds could never keep a secret from the Klingons for long, just take project Genesis....
I made a version of the L7 years ago but maybe a FoaS take on the design would be interesting.
I've also noticed that there are 3 versions on the D11 "One Wing" cruiser as the original build plus rebuilds / refits are also included in diagrams online.... Another future FoaS project??