The thing I want most from this game, more than anything else, is the ability to build my own vehicles. Maybe have it where you select what you're going to build so it can give you the right bricks like "Boat, Submarine, Plane, Helicopter, Car, Tank, Crawler" or something along those lines, maybe the ability to crossover to make a car-plane, or tank-boat too. Then when you have your vehicle-of-choice chosen it'll give you thrust pieces or wing slabs. Ever since Kalphiter released that Box Train addon that looked perfect to build upon I've wanted a Lego thing where I can just build and mess around with my own vehicles.
A first person mode would be nice as well for exploring interiors, as Blockland has proven. This game looks like it has a lot of potential and I'm excited to see what it's like when it hits full release.
But of course I'm buying it the very instant I have money in my Steam wallet.