Could you make so worlds aren't limitless, so you'll have to think twice before burning anything, but make a core, full of lava and "impure iron blocks" which you would have to purify to use, and in the depths of the different worlds you would find huge underground worlds and new ores and stuff like uranium and new minerals. And in different planets will be new ores. But with new ores and areas of building comes a price. As you get deeper, there is more heat, and eventually, you will need to craft a drill, and you will pilot the drill like the rocket, and there are different drills that you will need to go deeper. And when you go far enough down, you start coming up the other side! Water will flow down through the hole if you come up in an ocean, so you will need to craft a special radar to sense lava and water above or below you. To keep your drill safe. And you would use the ores found in the ground to craft rockets to go to other solar systems.