To be more serious, the possibility to integrate small pieces like RCS into a fuel tank would be really cool to me.
For example, see that launcher:
I've put RCS on my main stage, as I want it to be maneuverable, but because of the boosters, they can't be covered with fairings. The problem is limited in this case (I'll "just" suffer from drag), but in some situations, this can seriously complicate your design, especially when you design a massive rocket, and each pixel counts, as the grid size is limited. So it would be cool to be able to integrate them into the fuel tank like this:
This would also allow to better mimic the real launchers, that don't have so prominent attitude control system.
This could also be done with landing legs, which would allow to reproduce the Falcon 9 (its landing legs are not apparent!) with more realism, or to design a Saturn 5 without having the LEM legs messing with the slick design of the rocket.
It could be made at the cost of a fuel penalty (as this takes place from the fuel tank) or something like that to compensate, so that it's not a no-brain choice.