There is many gaps in the FS market as far as aircraft go, but the biggest gap for me is the short haul aircraft. There are plenty of mid/long haul planes out there, albeit some better than others(CS has three good ones) I think the the regional aircraft have been seriously neglected though. I can't speak for everyone but I prefer a flight that is an hour to two hours long and regional planes are perfect for this. While the early 737's would be my vote on the list I would prefer any of the aircraft listed below:
-Embraer ERJ-145
-CRJ-200
-Fokker F.28 series.
-Bae/Avro RJ series.
-Dornier 328 jet.
Just my opinion, non of these planes are well represented in FS.
-Todd