I think CaptainSim does best with classic airliners. The market is much too overfilled with modern stuff and planes like the 727 are just fun. I am not always in the mood for FMS and glass cockpits.

I also think a Tu-154 (m) would be great as there is almost no russian plane out there.

Also I would like to see a DC-10, because its just beautiful and the CLS version is a bit outdated and adapted from FS9. Any classic 737s are missing for FSX too, and if you do not want to go with THAT much work after the 727 and 707, try a 737 200ADV.

In fact, the visual quality of CS planes is unmatched and I'd like to see any plane with this.  ;)