Ability to connect multiple rocket engines to one rocket fuel tank.
Rockets are cool, I love rockets. However, they're a bit... Rigid. I've played a lot of Kerbal Space Program and you always had ways to make things work. A way of balancing things. Say, you make a ship that incorporates rockets. For example 3 M rocket engines connected to 3 M rocket fuel tanks, but that doesn't give you enough thrust OR you feel like the fuel drains too fast. Solution? Make your craft weigh less to compensate, or add more M fuel tanks and M engines. What if you don't have more room on your ship design? None of these solve the fuel drain. But what if you could connect several rocket engines to one tank? You would have a lot more freedom of balancing your thrust to weight and fuel drain. Imagine connecting several M engines to an L rocket fuel tank so that they perfectly fit your TWR needs AND decrease your percieved fuel consumption because you have a larger tank now, compared to using one L engine with way more thrust than you need, draining fuel much faster.