Currently, flight ratings take into consideration an aircraft's "Popularity with Passengers," and while most will agree when I say that the garden variety passenger would not care what kind of aircraft he/she is flying in, it's still a somewhat effective system for balancing out competition.
However, I believe that the current -5 to +5 bar ratings are a bit broad. And while I did mention that real life passengers wouldn't care about the aircraft type they are flying in, I believe that they would at the very least notice the space offered by widebodies compared to narrowbodies especially on long haul routes.
I propose a system similar to cabin configuration ratings which are based on flight distance. For example, A 73J would still get +5 bars for a 2,500 km flight but would only see +3 bars for a 7,000km. flight. On the other hand, an A333 would see +5 bars for both routes, since it is a widebody and is (perceived) to be more comfortable.
What are the advantages of this system:
1) It incentivice the utilization of more widebody aircraft, as that in real life airline ops. The 73J currently holds somewhat of an advantage for long haul routes, leading to poor sales and utilization of widebody aircraft in some servers.
2) It would balance out ratings for "niche" aircraft that fly to destinations usually with short runways. For example, a DH3 flying from MNL to MPH (which has a 950m. runway), a flight around 300km. should see at most a +3 rating, given that it is only a short flight and that there isn't really much of a choice other than the dreaded L4T. If the DH3 would fly an 800km flight, its rating should start to deteriorate as well in order to mainain balance.
I don't expect a quick implementation of this system as it would add a fair degree of complexity, but I believe that it could be done in the medium-term, especially since there is a template based on cabin configuration/service profile ratings which already have distance-based variables.