Hi,
one factor for the rating is the popularity of the aircraft which is always "constant" (high) and (in my oppinion) just a rough basic number.
Why not add a "national" factor? I think if an airline would grow in the US with a majority of russian or even chinese airplanes, their image would be lower. Or else: if you play in Brazil, Embraer would maybe give an increase in image over Bombardier or others..
Short:
Give an "bonus" for popularity for the country (or countries: Airbus :/) of origin for the plane manufacturer...