Salute
The flyable aircraft I think will be dependent on what info is available on cockpits, and how exact GT is going to be.
I did quite a bit of research on aircraft of this era, I know there are problems with getting exact cockpit details on a lot of these aircraft.
The aircraft I know cockpit details are available through originals or historically accurate reconstructions in museums are the following from the list Fly submitted:
FRANCE
Nieuport 17/17bis/23
Nieuport 28
SPAD VII
SPAD XIII
Breguet 14
GERMANY
Albatros DVa (DIII cockpit would be much the same)
Fokker Dr.I
Fokker D.VII
Pfalz D.XII
Halberstadt CL.IV (CL II cockpit should be much the same)
Rumpler C.IV
AEG G.IV
GREAT BRITAIN
Sopwith Pup
Sopwith Triplane
Sopwith Camel
Sopwith Snipe
RAF S.E. 5a
Bristol F.2b
RAF F.E.2b
RAF R.E.8
Airco D.H.4 (only American liberty engined versions still exist as far as I know)
Airco D.H.9
There is also an original LVG CVI two seater German aircraft at Shuttleworth.
http://www.wwi-models.org/Photos/Ger/LVG_CVI/index.htmlI am surprised GT is not considering modelling it. It was a common German two seater during 1917-18, also used by the Austrians.
Also there are several orignal Sopwith Strutters in museums. The Strutter was a common British and French two seater in 1917, equipping the majority of the French two seater escadrilles. Originally used by the British as a fighter in 1916, and also used by the Belgian Airforce 1917-18 and Imperial Russian Air Service and Soviets in 1916-1919. I'm surprised this aircraft is not also included in their list, considering it also used the Clerget 130hp engine which is being modelled for the Camel.
Here is one being rebuilt at Memorial Flight:
http://memorial.flight.free.fr/Strutteruk.htmlAnd an originals in Paris, Belgium and at the RAF museum:
http://www.wwi-models.org/Photos/Bri/SopStrutter15/index.htmlFor a late war Uber British two seater, (did 140 mph) there is the DH-9a, shown at the RAF museum: (bottom of page)
http://www.wwi-models.org/Photos/Bri/DH9a/index.htmlThe 9a was a DH-9 with a Rolls Royce engine, the orginal DH-9 was underpowered and a horrible plane.
For Fighters, there is an Austrian Oeffag Albatros D III at Memorial Flight if they want to do a Italian Front expansion:
http://memorial.flight.free.fr/Albatrosuk.htmlAnd an Austrian Phonix DIII at the Swedish museum:
http://www.wwi-models.org/Photos/AH/Pho_DIII/index.htmlAn Italian Ansaldo A-1 at the Bergamo museum:
http://www.wwi-models.org/Photos/Ita/Ansaldo/index.htmlThis was designed as a fighter, but ended up being used as a fast single seater bomber.
Hanriot HD-1. Used by the majority of the Italian Fighter Squadrigila by late 1918. Also used by the Belgians, including their Ace of Aces, Willy Coeppens. Originals at the Musee de l'Aire in Belgium, and at the RAF museum in Hendon.
http://www.wwi-models.org/Photos/Fre/HD1/index.htmlAnd an Italian heavy Caproni Ca-36 bomber at the USAF museum in Dayton. The Caproni was also used by the Americans in Italy, and the French in France as a heavy bomber. The Ca-36 is a late war aircraft, but very similar to the earlier models.
http://www.wwi-models.org/Photos/Ita/Caproni/index.htmlHopefully we will see all these sometime...
Cheers Buzzsaw