Fokker F-50 and F-70. Currently we only have the Fokker F-100, but it's little brother the Fokker F-70 is missing as is the popular Fokker F-50 turboprop.
For startup airlines, the Britten-Norman BN-2 Islander and the De Havilland DHC-6 Twin Otter would also be highly appreciated. There is too little choice in small affordable aircraft in the game.