1. Torpedoes- How many times have you wanted to build a torpedo bomber, but have to use bulky manufactured torpedoes? Exactly.
2. Turrets- so we can build bombers. Some as ball guns, dorsal guns, tail guns, waist guns, and flexible guns.
3. Curved fuselages- self explanatory
4. A better way to build floatplane hulls- they would be a "V" bottom instead of flat.
5. Flares/chaff- to evade missiles
6. Electric systems- solar panels, batteries, electric propellers, engine starters, and ion jets; we could fly forever, and for night, we can run on solar charged batteries.
7. Guided bombs and bombsights- bombs are hard to aim, hense the need for smartbombs, and for older planes, bombsights.
8. A higher powered minigun- like the Avenger on the A-10, it can rip through armor.
9. Armor- prevents damage from gunshots (also maybe self sealing fuel tanks)
10. Helicopter parts- rotors (main and tail) and some way of stabilizing them, so they won't go out of control
11. Tank tracks!- for tanks (or the KT flying tank, which is a plane)
12. Train tracks- for all the people building trains
13. Rocket motors- for making JATOs, missiles, and rockets
14. Boat propellers- for making boats!
15. Arrestor hooks- for carrier landings
16. Fuel leakage- for when your plane is shot
17. Realistic airfoils- to make better looking planes
18. Artillery- to make gunships!

Upvote and/or comment if yo agree (or even if you don't) As always, have fun!
-Gemista