excuse me what? The 732 “not well known”? Are you serious? It was one of the most reliable planes of all time! Everyone knows about the -200.
You have a good point. Now yes making new aircraft is hard or long while modifying an already available aircraft in the sim can be remodeled to the other variant. (In a nutshell) Jet-Pack said that creating the newer 737 such as the NG and Max would basically be making a brand new plane. The 736, 734, 733, 732, and 731 would be easier to model because they have similar structures. Now the 737-200 would be the easiest as the 737-500 is basically an upgraded 737-200 because it is similar in size and capacity.
Aircraft such as the A320 or 777 (Not including the A320NEO and 777X) and the would be easy too because they have models that barely have any change accept for length or just slight changes to the power plant and wings or other elements. 747 and 737 would be harder as they would need to re model the multiple models Boeing has made over the couple of decades the variants have been flying.