How about still born concepts or evolutions of such, i.e initial missile ship ideas that ended up very different when they eventually arrived, or concepts that were killed by changing technology, treaties and or real experience demonstrating a better way, i.e. cruiser and battleship carriers. Large cruisers / battlecruisers carrying not just aircraft but motor torpedo boats, post WWII Cruiser/Destroyers, or the station cruiser replacing station carrier for empire security and trade protection. Maybe aircraft concepts of what was thought was going to happen but didn't, i.e. the BP ads in aviation magazines from the 60s showing civilian inflight refuelling tankers refuelling concords in flight. Another idea is the land marines concept from WWI, i.e. armoured infantry that was assumed would evolve along side tanks but didn't happen until much latter.
I think those are all really good and acceptable ideas. Ive been thinking about this a lot and even consulting with others on the sight, and I think about the only real "limit" on this GB is the notion of time and eras.
So for example put yourself in the shows of a World War II aircraft designer and he thinks about how an airplane would look in 20 years. You can't really be "wrong" . its just an educated guess on what
his educated guess would be. Your example using ships is a good one. in the mid 1930s someone would probably be guessing the battleship is still the king and would be for the next 20 years, only getting larger and more powerful, rather than going nearly completely out of business.
And I think original ("pre evolved") concepts are a big part of that so definitely acceptable. I guess you have to kind of have the knowledge of styles and trends and features from different eras. If you are going to build a World War II F-18 hornet, it would probably have rounded wing tips for example, wing cannons, ordnance from the era, etc. Again you can't really be wrong I'd like to see lots of details to really back date it, but that's up the creator/builder/engineer and what they note and decide to emphasize.
On that note, its ok to take a subject and transfer it BACKWARD:
in this case you have a modern subject (the super hornet not the Prowler!
) going "retro" with its paint. As opposed to an official 1940s design:
I mean look at that JU-287! Streamlined, jet engines, Forward swept wings, and SPATS!? (I know that wasn't the intended design, but its a great look at the "original" prototype as mentioned above.)
So to break it down really simply, In this GB you are basically either backdating a modern design or taking an older design and modernizing it, or taking an image or idea from the time about the future and replicating it.
Hope that helps. Like I said its rather loose guidelines.
Happy modeling!