Vehicles with guns on them are really easy, it was one of the first add-ons I made almost immediately after buying Blockland
Whats hard is to make a helicopter that flies how a helicopter should - can hover in place, press keys to go straight up and down, tilting toward a direction to go in that direction. I have yet to see a helicopter that does this - they all just fly like a plane - going straight in one direction, and falling if you don't keep moving forward. I could be wrong though, I never really watch the add-ons section - just to download well known stuff or stuff that I suddenly realize I need, or if I want to check if something already exists before making it myself.