Variants of existing aircraft would be easier to make and they are still wanted by a lot in the community, for example:
1) Adding the constant speed prop variants for the Spit Mk.I and the 100 octane fuel variants for all RAF fighters. Keep what we have for battle of France scenarios since we have the map to use it.
2) Adding the 110 variants with the uprated engines and correcting the MG-FF ammunition and high speed behavior for all 110s.
If i had to go for AI-made-flyable my guess would be between a Do-17, Beaufighter or Wellington.
If i had to go for brand new ones, i'd be a bit imaginative and say Swordfish. Huge versatility with some very nice loadouts, useful for any future Mediterranean expansion, we can recreate some very famous historical missions with it (Taranto raid, sinking the Bismarck, unsuccessfully attacking the Scharnhorst and Gneisenau during the channel dash), it kept flying in the ASW role until the very end of the war and to put it all into context for what we currently have in the sim, it was active in the skies over Dunkirk during the evacuation.
Flying so slow that fighters have to drop gear and flaps to stay behind you, relying on stealth and maneuverability to survive, yet having a choice between torpedoes, bombs, flares and (in the later marks) rockets and radar, while at the same time being an open cockpit flying coffin...don't tell me that doesn't get you all pumped up