Straight to the point.
Elements that I think would really take this training software to new heights and enhancing those synthetic simulation systems using this product would be the following:

Elements that affect flight
1. Airport/Runway data
2. Navdata
3. Water dynamics for ditching training (good idea seen in another post by someone else)
4. Aircraft dynamics, Weather dynamics (including icing), Drag, thrust, climb angles, and resulting aircraft performance on these factors combined with weather changes. Maybe even on the fly modeling of dynamics with aircraft and environment to adjust to realtime changes that normally occur in the flying environment between all these factors.
5. Wake effects when behind large aircraft if you're flying a GA aircraft and the resulting problems that occur with this.
6. Improved commercial landclass data to extend the possibilities to make VFR reporting much more realistic.
7. Terrain enhancements to accomodate for elevation changes for airfields.
8. Coding that detects what time zone you are actually in (it might query a time server somewhere or local pc, or by Ip address) so when combined with weather system you will have the (current) day/time reflected in the sim that is what you see when you look outside the window.
9. not sure how to put this one... so when it's wet the aircraft can loose its grip by breaking too hard etc...
10. ATC corrected and somehow made to meet Aviation Authority rules per country like updating maps with control zones, VFR, military areas and ATC in the software letting you know you have busted the control zone in your country for example.
Lower down the list:
11. A better shading engine to improve time of day lighting.
12. A better way for the engine to deal with 3d clouds and not texture based ones (being 2d) and somehow reduce the framerate so that you can have weather changes over time reflected dynamically (really seeing those clouds float past you going as fast as the current wind dictates.

the remainder of my ideas will be eye candy features and i will leave those to another post.

These types of features I think are so important because they are core elements to flying and if software doesn't (eventually) have this stuff then it will only be a game flawed by reduced immersion and reality. I'm sure you programmers are the best in the field and have the passion to develop something that will be state of the art to the flight training world in the context of software development. I hope you really pull the pins out on this and create something more than outstanding and exeptional.

Kelvin ...

I have to second your last item. The quality of the existing one i would suggest lags so far behind the capabilities inherent in up-to-date sound hardware/software as to render it prehistoric ...

Alan

I think the current priorities should be, in the following order, (over time) fix the current issues, add new features that enhance the functionality of the simulator, then improve visuals. And yes, the add-on developers are the ones who make the sounds. The current sound engine is just fine IMO.