It will be really nice if in the folllowing updates of the game we have:

Laser weapon:

A customizable laser with its power in Watts [W], color, energy available [kWh] (duration of the laser in time), the thickness of the beam and reflection (how much the laser deflects based on the smothness of different surfaces of vehicles)

Guided Torpedos option (ON/OFF Button):

Just adding the option for torpedos to track an objective (basically the same as missiles but in water)

Proper camo paints/patterns:

Just a simple way to decorate vehicles. For example, instead of just one flat color, combining more selecting between 2,3 or 4 styles.

Some kind of multirole emitter:

For making functional radars/sonars or electronic warfare devices. Fully customizable all their parameters, such as range, power, frequency...

Water props/engines:

Similar to the current ones but actually working underwater, needing only fuel and sometimes air.

Rocket engines:

I know, this is SP no SR2, but will be nice if added.

Better lights and beacons:

Some gizmos that actually light the environment with a visible beam, choosing the power, luminescence...

Tracks Mod:

Adding the tracks 2 or a improved vesion of that mod as part of the vanilla game.

I know that many of these are kind of difficult to implement or require a lot of time to create, but I leave it here anyways, as simple suggestions.

(All images below extracted directly from Google Images)