OK, I used to play tekkit classic (before the classic was part of the name), and there were many things in it I would love to see
Here is my list:
-Nano/Quantum Armor
-Iron Furnace
-(real EE (phimalar is working on this, but the condenser was cool))
-jetpack
-a real wiki (the current one is very incomplete)
Also, where can I donate to the people who make tekkit (not just individual modders?)