The market approach trying to know what is mostly wished is the good one.
It will be difficult to be successful making a xxxth A320 or B737.
BUT must be completed by your own wish; "what plane doy YOU like more ammong those missing in the FS world.
An FS plane is not only a product but also an art piece, good results need that you enjoy doing the job. If your dreams are populated with warbirds, would you enjoy building an heavy airliner ?
However take a look on the Italian, British and French aviation history. There is a gold mine there from the smallest to the heavier remaining to be modeled for flight sim.
I take time to collect and post some picture and link them in a futher post.
But my current sign below show you already something I did not find yey for FS2004...