I LOOOOOOVE airships. And the "airship pilot" is definitely an archetypes in certain games. This could work...
They could have some "heavy weapons" (... rockets? a lightning gun?), the kind of weaponry the airship would have.
But this is not just a "one subclass and you are done". This requires its own setting.
Agreed. Definitely proficiency with all shipboard weapons. Maybe it also lets you boost or recharge other people's magic items, and the airship rules let you access other parts of the ship in terms of casting spells and using magical class features? So like, as the engineer or at the helm, you can boost the power to the forward guns, or to the propulsion, or even spend a charge to let the wizard cast shield around the ship?
I'll have to think a lot more about what should be in the subclass, what is in the ship, and what is in stuff like magic items and feats and spells,.
so an ace pilot with a custom ship yes?
I think that the custom ship is more a general thing, tbh. Possibly different subclasses for different classes, even. A Fighter subclass for aeronautical marines, a rogue ace pilot, etc.
AND/OR! What about infusions that are special ship systems? Maybe part of the airship rules would be that there are a couple built in infusions, that the engineer controls, so you're not using all your infusions known on your airship?
Would give a basic sky ship as part of the class be too powerful? Because lets face it it's the base of what this idea is about.
I think that a gliding armor suit would work, as well as the ability to boost or recharge magic items. Then, airship rules that treat different parts of an airship as magic items. and the subclass is immediately better at being an airship helmsperson or engineer than anyone else is, and the class features aren't useless in an adventure on land.
The armor I'd treat like the flying armor from the Elemental Evil adventure, but perhaps at will or at least rechargeable with time or access to an airships "core crystal power system".
While this is a good approach, the simpler approach would be to make the Aeronaut a background, not a subclass.
Skills: Perception, Nature (you need to know it to adjust for weather, understand natural threats to your ship, etc...)
Tool Proficiencies: Vehicles (Sea/Air), Jeweler's Tools
Background Ability: MASTER OF THE BLUE SKIES - When piloting an airship through the air, you can reduce either the crew required by 25% or increase the velocity of the airship by 25%.
Tieing too many class abilities to the airship limits the utility of the class in the typical dungeon setting. Thus, putting it all in a background allows you to build a more rounded character and get the flair of the airship captain.
I'd make that background represent any airship aeronaut, but simpler isn't really the goal, here. Satisfying is.