Here is my list:

 

1) Tire burst during a rough landing or during take off;

2) Damage report (for example, structural damage after a rough landing);

3) Random inoperative instruments or equipment before dispatch (MMEL);

4) Weather uplinking;

5) Tail strike damage simulation;

6) Realistic gear struts and better wing flex animation;

7) Better windshield rain animation (more blurred during heavy rain)...

8) Aquaplaning or reduced braking effectiveness on wet runways...

9) Take off performance depending on runway conditions (wet or dry)...

(3) There is already service based failure.

(7) That would cause big performance hit, which is why almost no developer does this effect.

(8) In FSX it's already more difficult to brake on wet runway.

(9) Go to Takeoff page 2 in FMC. It's there already.