(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?

This vehicles rules are meant to replace and improve the mounted combat rules. Dnd5 rules do not cover vehicles well and mouting rules have few issues: if mount and rider have a different initiative fights can be impossible; intelligent mounts cannot be ever be controlled; the reach when riding is unclear…

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.

Vehicles are willingly creatures (possibly constructs) that can be controlled by pilot inside them or controlled by a rider on them.

Riders are not protected by their vehicle, but they enjoy increased speed. Pilots instead are fully or partially protected by the vehicle surrounding them.

The first kind of vehicles are mount, the second are enclosing vehicles. E.g., a dragon, a hovering bike, or a flying skateboard are mounts; a mecha or a tank are an enclosing vehicles.

Normally, we assume all characters have an universal driving license. Checks should only needed for difficult or exceptional circumstances. You can add your proficiency bonus if have a proper Vehicle Proficiency.

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.

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

Similarly to the original rules, this is the only way to change initiative in dnd5, it may bring strange situations like a player that have to do his turn interleaved to other player turns or a tiny abuse to lengthen effects that lasts “until the end of the turn.” However, unless gameplay proves otherwise I assume it keeps the game fun and it does not need extra rules.

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 largest creature 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 on a big creature, for example a medium human on a gargantuan ankylosaurus, and it is surrounded for this rule allows to attack quite far away enemies. But without facing rules I think it is a necessary evil.
• 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 you succeed the saving throw you are still on the mount, but have the prone condition.
• 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 direct the mount using one hand. To be able to be directed properly some creatures may require specific equipment like bridles and saddles. Besides, some creatures may receive directions in different ways. For example a Dragon will perfectly understand speech; 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 follow your directions. 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. So, for example, to direct the vehicle to do an Action you use your own Action.
• Enclosing vehicles protects you, the exact effect depends on the vehicle (see the next two bullet points). When the vehicle reach to 0 hit points it is destroyed and any excess damage carries over to you. 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. For example, if you take 11 damage in tank that has only 4 hit points left, you take 7 damage.
• Partially enclosing vehicles takes half damage (round up), the other half (round down) is taken by the pilot. For example, if you take 11 damage in partially open mecha, the mecha takes 6 damage, you take 5 damage. Had the mecha less than 6 hit points then the excess damage would go to you as normal.
• 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 proper vehicle proficiency you can always use it.

Up to now we assumed the driver (or rider) was the only creature on the vehicle and rules focused on the interation between the vehicle and the external world.

However, depending on the nature of the vehicle there might be space for passengers.

Passengers are treated at the same as drivers, but they cannot direct the vehicle and in the case of partially enclosing vehicles who takes the damage after an attack to the vehicle is randomly decided.

Some vehicles may have multiple driving control posts; if two different characters try to direct the vehicle at the same time then the proper skill check contest decide witch one is understood. If the contest is a tie the vehicle understand both; what happens depends on the situation.

For example: Jozan and Tordek are sitting in the two control posts of a plane. Tordek is driving and Jozan acts as a passenger. Suddently, Jozan understands that Tordek is actually a doppelganger and it is flying torward an enemy base; Jozan takes control and they do a contest skill check on Vehicle Planes to decide which one of the two gives direction to the plane.

If a battle happens on a mount or inside a vehicle then it is necessary to know the exact position of each combatant inside the mount or vehicle space for it will not be enough to state that rider and mount share the space and that the driver is the vehicle.

When this happens consider that the vehicle can rotate only once at the end of his turn.

Enclosing vehicles may allow to use larger weapons. To know the damage we convert the dice using the following table. The damage changes are inspired by the Spell Enlarge/Reduce and the DMG rules about creating new monsters.

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.

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.

• Mecha. the vehicle is basically an extension of the pilot body. So it can use weapons, cast spells, manipulate foci… In addition of the mecha action any action the driver is able to undertake are possible via the mecha body.
• Form fitting. the vehicle is almost a second-skin; you can use your own Dexterity instead of the one of the vehicle.
• Hovering the vehicle ignores difficult terrain.

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.