As a pilot, it would be really helpful to know what altitude you're at and what speed you're going.
Those are pretty basic instruments for any vehicle that flies. But they would increase the immersion of the game and keep the pilot informed.
Right now you're pretty much only relying on visuals, which works, but you can't compare speeds between ships or see just how much speed you get from kerosene.
An altimeter would not help you from crashing into mountains or buildings, since it would be based on sea level.
But it would help you get a sense of where the ceiling is at and what rate you are descending/ascending at.
I'd also love to see an actual magnetic compass, in addition to the hud one. Because brass gauges and such really brings out the steampunk feel.
Note: The images are from Rise of Flight, depicting real WW1 instruments. Thus they are not that easy to read..
Here is something more easy on the eye.