1) A group of 30-50 seater airliners in a 1 X 2 seating configuration in one class. These would be similar to the embraer / canadair regional jets in appearance. Range would be around 1000-2000 nm. These would be a good replacement for some of the smaller turboprop and regional airliners.
2) A group of 70-100 seat airliners. There seems to be a lack of a common family of airliners in this seat range. Many airlines have something along the lines of a Dash-8-400, or a Canadair regional jet at the lower end of this size range, and full sized jet liners such as the smaller 737s and A318s. Airlines seem to be heading toward regional jet-type aircraft for anything 100 seats or smaller. These would be essentially large regional jets, but with a more mainline cabin feel. Possibly 2x2 or 2x3 seating, with the provision for a two-class configuration if necessary. Range would be around 2000nm, with the option to have an extended range version for thin, point-to-point transcon routes.
3) A group of airliners similar to the A32X/737NG family. A single family of aircraft ranging from the 73G/319 to the 739/321 has proven to be very popular. In fact, I wouldn't propose anything too different from the current aircraft available in this range.
4) A group of smaller widebody airliners ranging in size from slightly larger than the 752, to around the size of the 764. Seating in coach would be 2x3x2 or 2x4x2. Seating in two classes would be ranging from around 200 to around 300, or around 180-250 in three classes. (similar to the current A300/767 series). This would be offered in both a domestic, two class configuration, and a long-haul, three-class variety.
5) A group of large, long-haul, widebody aircraft. Ranging in size from the A330/777 to the 346/747. Seating would be 3x4x3 in coach. The smaller end would seat around 300 in 3 classes with a range of around 9000nm, while the large end would seat around 450 in three classes, and have a range around 6000-7000nm.
This lineup would allow the world's largest carriers to replace all of their mainline and regional aircraft types with only 5 types (3 mainline types). This would suffice for most airlines. However, there are two specialized types that would be required for the airlines that like to have the more impressive types in their fleet:
1) A group of double decker airliners similar to the A380 series. Seating would range from 500-700 in a three-class configuration. Seating would be 3x5x3 on the main deck, and 2x4x2 on the upper deck. Range would be from 7000nm-9000nm.
2) A next-generation supersonic transport. seating would range from 100 to 150 in a 2x3 configuration. Range would be 6000-8000nm.