as Samy has posted, that he wants to implement freight in the 1.3 version, I am noting the following thoughts about it:
In another online airline game, specialised freight airlines have big trouble in competing with regular airlines, as those can take freight as an additional item and can easily offer lower fares, than normal freight carriers. This can be avoided by offering different sizes of freight items:
Like mail (units of 0,1kg each?), packages(units of 10kg each?), aircraft containers (units of 500kg each?), heavy freight (units of 5000kg each?) and superheavy freight (units of 20.000kg each?). Mail could be loaded into nearly every aircraft, for packages You need already a larger freight room, like in an E170 or a DC-9, for aircraft containers You need at least a standardbody aircraft like an A320 or a 737, for heavy freight, You need already a specialised freight aircraft (like a 737-200F or a DHC-5) or a Combined freight/pax aircraft (like a 747-200CF). For superheavy freight You need a freight aircraft, like an AN-124, a Short Belfast or an Il-76.
Each airport should only offer and/or accept only some different kinds of freight. Mail and packages should be offered and accepted nearly everywhere, aircraft containers at all large airports, heavy freight only at airports with extra freight terminal, superheavy freight only at selected airports with heavy industry in the area.
There are two versions for filling an aircraft: Variant one would be, You can only fill the largest type possible for the route (example, an aircraft with 11.250kg freight capacity would fill two units of heavy freight or 22 units of aircraft containers, or 1125 units of packages), variant two would be, You fill it first with the largest freight, then continue filling it with the second largest and so on... (That would allow 2 units of heavy freight plus 2 aircraft containers plus 25 units of packages). That would be for Sami to decide, how complicated he wants the system.
The freight revenue should be higher for the larger freight units.
This is just a raw drawing and is of course subject to discussion and changes.