first of all: great work, thanks a lot - its a must when i play OTTD!
second:
some ideas on *generational upgrades*
Fokker 100: before the Fokker 100 there was the F28 and later the F70, both lower capacity though
A320/B737: maybe including the new generation of planes coming up? (A320neo/B737Max)
A300: making a distinction between -B4 and -600 models, with different range
B707: -120 and -320 distinction with different capacities (similar to the DC8-30/60 distinction)
B727: including the 150-seat 727-200 Advanced as an update somewhere in the late 60ies/early 70ies
B747: SP version could be interesting since there is a new range feature (exceptional range, less pax)
B767: upgrade to include the -400 version with higher capacity maybe?
B777: a bigger version maybe? a -300? or the LR model? also for the A340 there could be some differentiation possible
Bae 146/Avro: maybe possible to make two planes out of it with lower operational costs
ATR72: maybe have a 42 version ahead of it?
Dash 8: a generational upgrade with the Q-series?
some ideas on *additional planes*
40/50ies:
Sud-Est Languedoc (long range, low passengers)
Sud-Est Armagnac (a bit like the britannia)
Breguet Sahara (a really nice double decker)
Vickers Vanguard (medium range, big plane)
Convair 240/340/440 (small piston planes, similar in size to the viscounts)
F27 + generational upgrade to F50 (yes, similiar to the viscount, i know)
60ies:
B720 (a derivate of the 707 with shorter range and lower costs)
Vickers VC10 (long range, less passengers than the 707; (it looks similar to the 62, so maybe less work))
Hawker Siddeley Trident (like the 727 a bit; resembles it optically quite well, especially the 721 model)
70/80ies:
B757 (really useful as a longer range model compared to the 727/737)
Lockheed 1011 Tristar (looks like the DC10, just with the much cooler ducted rear engine)
DHC-7 (useful in the future with STOL characteristics)
Saab 340 & later the 2000
BAe ATP
90ies:
Canadairjet CRJ200 & later the versions up to 900
B717/MD90 (as a replacement for the DC9 or as an upgrade)
2000+:
Bombardier CSeries
Embraer E190
A350/B787: can be useful because of the expiring of airplanes, so that there is a replacement available
and of course all the russian planes, the most interesting of which are in my opinion:
TU-104, 114, 134, 144, 154, 204/214
Yak-42, SU-80, Superjet, IL-86/96, 114, AN-74, 148/158
and the future Irkut MS-21 and COMAC C919
oh... well, thats now a very long list. but glad about anything at all
third: changes to the system
my idea is that range should be penalized in aircrafts by raising the price accordingly or the running costs, so to prevent people to use the long-range airplanes so much. and it could be interesting if there could be a refitting of airplanes in different sizes (like the B737 Classic could be available as the 300, 400 and 500 etc.). but really, i have no clue about programming or coding and all that stuff, so i assume it must be a really big ammount of work and probably not worth the effort.