Vehicle Mechanics

Road Hopper
1
Artifact - Vehicle
Road Hopper's operators get +1/+0.
At the beginning of each combat, if Road Hopper has exactly one operator, that creature gains menace until end of turn.
Operate 1 (1: Target creature you control operates this vehicle and no other vehicle. Operate only as a sorcery.)

Horseless Buggy
2
Artifact - Vehicle
Horseless Buggy's operators have haste.
At the beginning of each combat, if Horseless Buggy has exactly two operators, those creatures gain first strike until end of turn.
Operate 2 (2: Target creature you control operates this vehicle and no other vehicle. Operate only as a sorcery.)

Sky Sloop
3
Artifact - Vehicle
Sky Sloop's operators have flying.
At the beginning of each combat, if Sky Sloop has exactly three operators, those creatures get +1/+1 until end of turn.
Operate 3 (3: Target creature you control operates this vehicle and no other vehicle. Operate only as a sorcery.)

The basic difference I'm proposing between Equip and Vehicles is that vehicles provide a secondary bonus if they are optimally crewed. But it's also possible to have too many bodies aboard a vehicle, so optimization is a bullseye rather than a threshold.

I also toyed with a range of numbers rather than an exact bullseye to discourage instant speed removal from making the secondary bonus too unreliable. I guess that depends on the power level of instant speed removal in the set. Anyway, those version look like this:

Road Hopper
1
Artifact - Vehicle
Road Hopper's operators get +1/+0.
At the beginning of each combat, if Road Hopper has exactly one or two operator, those creatures gain menace until end of turn.
Operate 1 (1: Target creature you control operates this vehicle and no other vehicle. Operate only as a sorcery.)

Horseless Buggy
2
Artifact - Vehicle
Horseless Buggy's operators have haste.
At the beginning of each combat, if Horseless Buggy has exactly two or three operators, those creatures gain first strike until end of turn.
Operate 2 (2: Target creature you control operates this vehicle and no other vehicle. Operate only as a sorcery.)

Sky Sloop
3
Artifact - Vehicle
Sky Sloop's operators have flying.
At the beginning of each combat, if Sky Sloop has exactly three or four operators, those creatures get +1/+1 until end of turn.
Operate 3 (3: Target creature you control operates this vehicle and no other vehicle. Operate only as a sorcery.)

Operate - Is this supposed to be a 1 to 1 relationship or can multiple creatures Operate the same thing. I dislike anything that is basically equipment with downside.

Board - I get that the creatures aboard a ship must attack together but what happens when one of them is blocked? (hopefully nothing special but that is still a little weird) There is no way to get off a ship without getting on another?

Personally I think I would just make some pilots that can tap at sorcery speed to boost target vehicle(or just artifact creature) for as long as they remain tapped. inspired by Ghosthelm Courier cycle.

I think effects like A are for the best. At higher rarities we can experiment with noncombat bonuses and other stuff like that. Maybe noncreature Crew could also appear as well at higher rarities.

Like I said, I think "can't attack or block unless crewed" isn't necessary as long as the art / flavor make it clear there's at least a few dudes on the vehicle.

Huh, that is an interesting way of doing it. Their inability to stack is intentional, but still feel-bad.

I think this is ok if it is going to be a small theme at uncommon and up. It has some issues that I would consider not good for common, I'm not saying it can't be at common because of these just that it seems not good.
-If your deck has too many they are worse than normal equipment. It is flavorful but we don't want players to just think that it is an old mechanic with a new downside. If we only have a handful of them we can make them all strong and most players that see the downside will also see all of the upside they get. I am just concerned that a player's first look could be an average strength common and they will see the downside first, making the whole theme unexciting.
-It does not matter how big your creature was. Usually we want new mechanics to make players think about what new thing matters, not about what old thing no longer matters.
-Timestamps matter. If Autocar and Autogyro are both common it is not unreasonable in some limited games for a player to want to make their creature into a 3/3 with flying, which they can do if they equip them in the right order. There is no obvious visual way to show if the creature is a 2/2 or a 3/3. (I am pretty sure but not positive that it is the order the 2 were equipped and not the order they were played)

On the plus side, I do like the idea of Pilots just being 1/1 creatures with good combat abilities. They would synergize with vehicles in a nice non-parasitic way.

I have to contest the assertion that these vehicles are a downside mechanic. Yes, they're not as useful on larger creatures, but neither is Bonesplitter. It's merely that the conditions that determine when they're strongest aren't the same as what we're used to.

Similarly, players running vehicles do still care about the size of their creatures, it's just that they want cheaper creatures to equip, rather than bigger ones. Or, to speak in the same terms, where we benefit less from a creature being bigger, we benefit more from a creature's other abilities. I'd rather put an Autogyro on my Stealer of Secrets than a Bonesplitter. Or an Autonaut on my Child of Night.

The timestamps concern is an excellent one. Equipping two vehicles to a creature will confuse players dangerously. I suppose we could add a rider to prevent a creature from equipping multiple vehicles, but that _would_ be downside, and bulky.

Of course, I've been fiddling with "equipment that tap" for the challenge of solving Combine, and I hadn't even thought of putting it to use here!

Sgorta Airship    {2}
Artifact - Equipment
T: Equipped creature is a 2/2 Thopter artifact creature with flying until your next turn.
Equip {2}

This (literally Autogyro + t:) means the timestamps are easier - whichever one you activate overwrites the previous, just like overwriting the base P/T. The flavor turns out a little weird if your creature jumps in an airship (2/2 flying) and then in a mechsuit (4/4) but keeps the flying... but that's no weirder than a Craw Wurm or Insectile Aberration wielding a Sword of X&Y, which happens every day in Magic games. 

Distinct to Tesla; uses existing familiar tech; interacts with Pilots, Canisters, and +1/+1 counters - and there's still room for flavor text.

Sgorta Airship {2}
Artifact
1, T: Target creature becomes 2/2 and gains flying until your next turn. Activate this ability only as a sorcery.

Airship {2}
Artifact - Equipment vehicle
Equipped creature is 2/2 and has flying.
When you equip CARD unattach all other vehicles from equipped creature.
Equip {2}

If we want a simple equipment, how do we feel about a triggered ability? I think this is better than making a new keyword that is similar to equip. It is flavorful, and even new players should be able to play it without having to think about timestamps (assuming we don't print something like unforge that may want to be cast in response to this ability). It is a bit of a pain for MtGO players that have to pass priority on a trigger that usually does nothing.