Just like in Take on Helicopters or flight simulators like X-Plane 10 or DCS:Warthog you should be able to interact with the systems of the vehicle you are in manually , to either start the vehicle in a proper manual way or interact with systems like lights,hud´s,mfd´s or other systems through interaction with the mouse on knobs,swithces or touch screens.
This would allow modders to create authentic replications of aicraft and other vehicles and to make them more immersive.
Controlling the systems of a vehicle by the scroll menu is a big immersion braker and to interact with these long menus is very cumbersome.
Instead vehicles with many functions can now be interacted with through a much more natural way.
If you don´t want to implement this into the ARMA 3 stock vehicles at least enable the interaction of vehicle cockpits with mouse for 3rd party addon vehicles.