Might be working on a sentient vehicle class. What flavor should I go with?

[Greywander]

I get the most random inspiration sometimes. Anyway, I was thinking about making a class that eventually turns your character into a gargantuan vehicle that the rest of your party can ride, with firepower to match. Likely with three subclasses: a burrowing tank, a submersible battleship, and a flying fortress. Since you'd expect such a vehicle to be much stronger than any of your party members, I'll probably balance it out by making you roughly on par with them when operating independently, but your party members can man turrets and such to bring you to full power. It's an interesting concept I haven't seen explored like this before. There's a lot of potential issues that will need to be worked out, but before I start tackling the exact mechanics, I was wondering about what flavor I should go with.

Currently, I have three potential ideas:

You are a construct built for war (e.g. basically a super warforged). Whatever race you choose is refluffed to just be variations in your design giving you some extra bonuses.
You are a pilot for an evolving mecha. This might involve turning your armor into a massive vehicle and back. So you're not really playing the vehicle so much as the pilot.
You are a spooky ghost that has possessed a warmachine, and are now expanding and improving it. Here, your racial traits make sense because you actually were that race in life.

There might be some leeway for me to leave it open-ended so that you can pick and choose what flavor to use, but I feel like playing as the pilot would necessarily be mechanically different. Namely, in that you should be able to exit the vehicle and walk around as just a normal person. Playing as a sentient vessel sounds more fun to me, though. What are some other ways I could handle it, though?

Edit: It occurs to me that the conceptual basis of this class gets even trickier if multiclassing is on the table. Hmm...
Edit 2: I could have sworn I was posting this in the 5e subforum, but oh well, Homebrew makes sense, too.

[Old Harry MTX]

I like the spooky ghost idea, but if you want, take a look at my Pilot (https://docs.google.com/document/d/1DT2ssmyao_HlmSydGWLlLyhlIRJ6qXnNW7WglCj7fZE/edit?usp=drivesdk) here.

[Greywander]

Looks interesting, thanks for sharing. It's quite a bit different mechanically than what I had in mind, though.

Maybe I can just offer the player a choice at 1st level:
Mechanical transformer: You are permanently locked in your vehicle form.
Human-hybrid transformer: You are an otherwise normal person that can somehow turn into your vehicle form.
Pilot: You summon the vehicle instead of turning into it.

Alternatively, I could just go with the first option in such a way that you can fluff yourself while in medium-sized vehicle form as either a person with a shoulder cannon or as a small tank. Though I was thinking that while in vehicle form you would count as a construct and not require air, food, drink, or sleep, but maybe those should be left to your choice of race. Warforged get to just be a straight up construct (though technically humanoid), and reborn can do the ghost in the machine pretty well.

Something else I was thinking about was how to handle the turrets. Some classes, like paladins, get bonuses for using certain kinds of weapons, e.g. melee weapons. What I was thinking was that you could insert a weapon into the turret and it would take on the properties of that weapon, essentially allowing that weapon to be used while someone is riding you. The turret would apply a slight range and damage bonus, with a higher bonus at larger sizes. Mages can also cast spells through the turret to get the same bonuses. What I wasn't sure about was what to do with melee weapons. I could set the base range of any weapon to be at least the same as a thrown dagger, 20/60, and make it still count as a melee attack (e.g. for smites), but that's a bit strange conceptually. I could make it some kind of actual melee weapon with an extended reach, but having an actual arm sticking out the side of your tank/battleship/airship swinging an oversized weapon seems a bit too goofy.