(Paolo) Shamans have the ability to transform in their spirit animal. However the ability does not scale with level. While it is true that most racial abilities do not scale, I see this ability akin to the Dragonbonrn's breath weapon. What about adding something like:

To the stat block of the animal?

(Paolo, Work in Progress) Issues: how to treat passengers; tidy up.

Vehicles rules are meant to replace and improve the mounted combat rules. Vehicles are creatures (possibly constructs), but depending on the vehicle they are basically normal mounts or a second skin just like Druid Wild Shapes. Vehicles that behave like a second skin and protect the driver fully (exactly as a Druid Wild Shape) or partially à la Shield Guardian.

We call this kind of vehicles: mounts, fully enclosing, partially enclosing. E.g., a dragon, a hovering bike, or a flying skateboard are mounts; a Mecha can be a fully or partially enclosing vehicle depending on the model; a war tank is a fully enclosing vehicle.

If the vehicle allows to use larger weapons we convert the dice following the table. The damage changes are inspired by the Spell Enlarge/Reduce.

For example, let us think to a Large Mecha robot with a Battleaxe. Battleaxes normally deals 1d8 damage (1d10 two handed), in the hand of the Mecha the Large Battleaxe will deal 1d12 or 2d8.

What is a vehicle or a mount? In dnd5 the rules state: A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount. There are no rules for riding unwilling creatures and the “appropriate anatomy” is a bit fuzzy.

In the real world men ride ostrich whose size is Medium and in the classic fantasy we have dog riders, but dog backs would be probably too weak to carry a rider. Jeremy Crawford suggested a rough guideline: a mount should be built in such a way to comfortably bear a rider for extended periods of time. Since we do not want to pass the whole monster manual to tag what is a valid mount, we remove the size limitation and leave it as a DM call.

However, if designing a new mount or a vehicle, it would be a good idea to state it clearly in its stat box. We remove the size limitation as some Sci-Fi mounts would be probably smaller than their rider, for example a flying Skateboard would be probably a Small creature able to carry a Medium rider. As usual particular beats general, this rules are meant as default.

Normally, we assume all characters have an universal driving license. But in case of difficult driving a check may be required. In this case if you have the Vehicle Proficiency you can add your proficiency bonus.

To enter or mount a vehicle, you need to use half of your movement and you cannot enter a vehicle if your speed is zero. For example, a Human needs to use 15ft of movement to enter or mount a vehicle. Dismount or leave a vehicle costs the same.

Vehicle and pilot initiative are the same: if the pilot enters, or mounts, the vehicle mid-fight the initiatives become for both the lowest of the two. If the initiative is rolled when the pilot is already inside or riding a vehicle, then the vehicle rolls for initiative and the result applies to both rider and vehicle.

When you entered or are riding a vehicle, the following rules apply:

• You and your vehicle are basically two creatures sharing the same space.
• Mechanically both use the same space, just consider that depending on the mount the rider might be in a elevated position so he needs to use a reach weapon to be able to hit. (e.g., an Halfling on a Elephant may need a lance to hit) It appears a bit ridiculous if the rider is surrounded, but without facing rules I think it is a good compromise
• If an effect moves your mount against its will while you're on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it. If you're knocked prone while mounted, you must make the same saving throw.
• If your mount is knocked prone or killed, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it.
• If the mount provokes an opportunity attack while you're on it, the attacker can target you or the mount.
• You can give commands to the mount using one hand. Some creatures may require specific equipment like bridles and saddle. Some creatures may receive commands in different ways, for example a Dragon will perfectly understand speech; a hovering bike won't need any special equipment a free hand is enough to give commands; a flying skateboard is controlled with the feet.

While you are on a mount, you have two options. You can either control it or allow it to act independently. Some intelligent creatures may not accept to be controlled and some creatures may act foolishly when independent. For example, domesticated horses, donkeys, and similar creatures are assumed to have been trained to accept a rider, but left independent they'd probably escape fights ignoring your commands. Similarly, mechanical mounts normally accept any driver, but while acting independently they do nothing.

Examples: a Dragon may be too proud to accept being controlled; a hovering bike may have an AI that follows commands obediently even if acting independently; a flying skateboard cannot act independently, so can only be a controlled mount.

A controlled vehicle moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it.

Bearing a rider puts no restrictions on the actions the vehicle can take, and it moves and acts as it wishes. It might obey your commands, but also flee from combat, rush to attack and devour a badly injured foe, or otherwise act against your wishes.

• You use the Strength, Dexterity, and Constitution of the vehicle, you keep your own Intelligence, Wisdom, Charisma and personality. Outside what is written in this rules, mechanically you are the vehicle.
• Enclosing vehicles protects you, the exact effect depends on the vehicle. But when the vehicle reach to 0 hit points it is destroyed and any excess damage carries over to you. For example, if you take 10 damage in tank that has only 2 hit points left, you take 8 damage. As long as the excess damage doesn’t reduce your hit points to 0, you aren’t knocked unconscious. But you fall prone in the vehicle space surrounded by the wreck.
• Fully enclosing vehicles protect the pilot completely, any attack will damage the vehicle Hit Points exactly as it happen with Wild Shape or polymorph.
• Partially enclosing vehicles takes half damage (round up), the other half (round down) is taken by the pilot. As in the previous example, if the vehicle has 2 hit points left and you take 10 damage then 5 hit points will be taken by the vehicle and 5 will be taken from the driver. Since the vehicle had only 2 hit points left the excess damage of 3 also goes against the driver.
• Your ability to be heard, take any action that requires hands, cast spells or using skills is limited to the capabilities of the vehicle. Entering a vehicle doesn’t break your concentration on a spell you’ve already cast, however, or prevent you from taking actions that are part of a spell, such as call lightning, that you’ve already cast. For example, a mecha normally allows to do all kind of actions; a tank allows only to make its own attacks.
• If you have the vehicle proficiency you can always use it.

Speed. Vehicles have the same kind of speeds of other creatures; plus few ones for Space Operas. Space Speed and Hyperspace Speed, those speeds are expressed as factor of the normal travel speed. E.g., if a destination is distant 10 space days, and your vehicle has a Space Speed of x2, then you will reach destination in 5 days.

Some enclosing vehicles allow the pilot to use some of his abilities.

• Hands. the vehicle has his own robotic hands or it is open enough to allow the pilot to use weapons, cast spells, manipulate foci…
• Form fitting. the vehicle is almost a second-skin; you can use your own Dexterity instead of the one of the vehicle.

Normally when a vehicle is destroyed the pilot is prone, but some vehicles have different behaviors:

• Death-trap. Once the vehicle is destroyed the wreck locks you. You are Prone, your speed becomes 0, and you cannot benefit from any bonus to your speed. To leave the death-trap you need to use an Action to pass a Strength (Athletics) or Dexterity (Acrobatics) check as contested by the vehicle Strength. If for any reason you have advantage (or automatic success) escaping grapples you can use the power here.
• Quick escape. Unlike normal ruling, once the vehicle is destroyed you are not prone.