This is exactly what I think. The 736 and the 318 are add-ons to their respective families, provided to airlines which already operate members of this family and have thus the advantage of cross-crew qualification, maintenance etc. As stand-alones they are expensice due to the already mentioned strucutral over-weight which increases land fees, fuel consumption and have loading problems.
The "New Generation" regional jets have been design for this market with the 70-seats (728JET, Embraer 170, CRJ700) being the front-runners. The follow-on models in the larger market around 100-seats are straight derivates and share a very high degree of commonality - but at a much lower weight. As many might know by streching a plane you usually get slightly improved performace figures and lower seat-mile costs, this is why Embraer has a three-strech (Embraer 195), Fairchild-Dornier has the 728JET streched once to the 928JET and is considering a so-called "1128JET" as the third family member.
Especially the 100-seaters are appealing to mainline-carriers as they are very suitable B737-200, Fokker F100 and 28-4000, DC-9-10/20/30 and BAe 146-200/300 replacements. The 70-seaters are of more interest to growing regional airlines who have grown out of their 50-seaters.
I see a big market for these new aircraft families, especially the Embraer 195 and 928JET will attract orders IMO, maybe already this year with the Qantas/British Airways, the Air France, the British European and the US Airways decisions pending.
In the long-term I see Northwest as a very likely customer for this kind of aircraft, better aircraft of the respective families. Why? The 120+-seat segment is covered by the A319/320, the lower segment till 70-seats is operated by the Link divisions. Leaves the 80-110-seagment wide open, THE terrain for the NG-RJs. I won´t comment on the B717-200 here, this topic has been discussed to death.
The regional plane orders site