Modding will have two updates in update 1.0-14.
"isSpecial":true variable that can be added to liveryData.json indicating that a livery is rare and should not spawn as frequently as the standard livery, current ratio is 33% (might be adjusted later on). This is only applicable if the airline has a multiple liveries of the same aircraft type.
"fleetCount": variable can be added to the mod.json file for each airline. This array of ints must be exact same length as the fleet type array otherwise it will default to 1 for each aircraft type. This will make aircraft types with a higher count spawn more frequently. Below airline has one A319, two A320 etc. Note that indexes must match!
"fleet":[ "A319", "A320", "A321" ], "fleetCount":[ "1", "2", "3" ],
Just so you know, existing mods will not be affected so this is completely optional.
Let me know if any questions