It would be cool to have some kind of tool which would show us where we should place RCS thrusters on the opposite side of craft's center of mass when placing the RCS thrusters (with an option to place them on both sides at once of course). This way we could make balanced RCS steering easily, which is important for craft that are meant to dock a lot - space stations, complex missions with separate transfer modules and landers, etc.
In KSP there is a mod that shows the torque generated by different trust types (engines / RCS) if I remember correctly. I don't know why it wasn't in the main game because it was a must if you want to make complex spacecrafts.
A tool similar to this will be a great addition to the designer.