add items:
- magnets
- Conveyor Belts.
- Gears & Cogwheel.
- bigger flying blocks.
- ancient rocket propulsion block
- light weight controls for aircraft
- soft braces works like rubber band
- smooth surface pieces for ballista railing.
- ability to build item diagonally, also add diagonal wooden joints
- circular spring mechanism like the following pic


add weapons:
- ice balls/ snow balls
- launch-able COW! or sheep which can explode
- human cannons ?!?!
- Speargun with ropes attached at the back
- ancient dragon rockets like the one in Disney's Mulan
- van helsing's automatic crossbow


Change on existing items:
- be able to customize hinge max turning angle
- be able to customize steering hinge max turning angle
- bigger aerodynamic propeller with aerodynamic spinning blocks
- flying wings which gives a bit of upward force when flying
- strong ramming beable to damage walls

Enemy:
- orc, large and small
- yeti (why not haha)
- Mage
- Shield guy
- enemy siege weapon
- princes, king & queens

Level design:
- icy level? so flaming balls have some use to it
- flying machine levels, with bigger building boxes, because adding wings costs alot of space
- Boats? would be fun to build our own boats and shoot each other with cannons
- PVP battles where each side have a group of soldiers or castle then + player's siege weapon, then duel it out
- now come to think of it, we need an chess level

after reading all the older posts, I found out some post already pointed out some ideas above, so ill highlight repeated ideas in blue.