I think the entire way fleet types are done is unreasonable. I feel like commonality cost should scale on a curve looking like x^(0.5), with some considerations with the type of aircraft in mind when adding types, just as size, engine, airbus/boeing, ect, and airlines should be highly incentivized to stay to one fleet type because it should be just so much better than two or three. This would make the game may be less favourable for new players as large airlines would have less incentive to stay to a few types, and adding 3,4,5... however many types would only incrementally lower margins, however I think it would be cool to have the playstyle having planes to fit your market perfectly could be fun too. Right now it seems pretty dull
with fleet types, stick to three, save cash before transitioning and never get the chance to BK, pretty boring. I would be far more awesome if for example if you ran a diverse fleet, say one player came along with only a single fleet type, therefore having far greater margins than you, and simply overlapped flights in that market so that you would lose money while they made a slim profit- That would be competitive and cool.
Basically what Im saying is I completely disagree with the way commonality is done, that 3.5 billion is complete bogus and makes no sense, even from a game balance perspective becuase it only hurts inexperienced players- not it intention IMO. If we want large airline to be punished, they should be punished for trying to cover multiple niches rather than a single in the best possible way, so for example a airline running both domestic and longhaul shouldn't beable to get the same margin as an airline doing just one of the two, therefore allowing playes trying to occupy niches to excell aswell, just like Ryanair did IRL in europe. RN for example I am a pretty new player, and I may have tried to do a fleet chance without stored cash, I would have no way of ever knowing my commonality cost was gonna multiply by ten, If i BK'd from that I doubt i would ever come back to the game- it makes no sense compared to real life. I wish the actual way commonality would be would be scale up cost with fleet types signficaly between one and two, and sigificantly between two and three, but less and less after that.