That wouldn't be a beast. That would be a dragon. Heck, if I introduce Sarkhan as an NPC, I'd likely portray him as a talking Red Dragon from the get-go. So yes, I can and will model planeswalker NPCs as I see fit to properly capture their character, but trying to understand them as if they were PCs helps the design process.

Since I've decided to exchange Wild Shape with a Totem Armor-inspired equivalent ability (or perhaps more of a beast-summoning ability and Totem Armor can be simulated via a Zendikar-exclusive druid Circle) and have Planeswalker NPCs have personalized features but claim to be certain appropriate classes regardless, what MTG classes would the PW cast be had they been creatures CURRENTLY? (for example, Nissa was a Scout before her ascension, but seems more like a Shaman or Druid now, though the Animists were treated as Shamans)

Ajani • Cleric/Wizard
Arlinn • Cleric/Druid
Ashiok • Wizard?
Bolas • Wizard
Chandra • Shaman
Dack • Rogue/Wizard
Daretti • Artificer
Domri • Druid/Shaman
Elspeth • Soldier
Freyalise • Druid
Garruk • Druid?
Gideon • Cleric/Knight
Jace • Wizard
Karn • ????
Kaya • Assassin
Kiora • Druid/Wizard
Koth • Monk
Liliana • Wizard/Shaman
Nahiri • Druid/Soldier
Narset • Wizard/Monk
Nissa • Druid/Shaman
Nixilis • Warrior/Wizard
Ral • Wizard
Ramaz • Druid/Shaman
Sarkhan • Shaman
Sifa • Wizard
Sorin • Soldier/Knight/Wizard
Tamiyo • Wizard/Advisor/Scout
Teferi • Wizard?
Tezzeret • Wizard/Artificer
Tibalt • Wizard
Ugin • Wizard/Advisor
Venser • Wizard/Artificer
Vraska • Assassin
Vronos • Soldier/Wizard
Xenagos • Shaman/Druid

As a quick aside, there are lots of people developing stuff for D&D Magic campaigns in 5th Ed.

-Temur Races and Classes
-Temur Spells

How should I go about leaving in Wild Shape for players who want to emulate Sarkhan's dragonic shapeshifting, but also be able to avoid it and allow only more traditional Druidic features and abilities?

Elaborate

Well, very few druids or even shamans actually *transform* into animals or magical beasts, and only on Zendikar is Totem Armor a viable replacement (while I like to begin with Zendikar, I also would like to structure things in a way they can apply all the time, not to mention while it's a cool mechanic I'd like to see on a character, it doesn't apply to better known druids/shamans like Nissa, even a Zendikar-native druid) so I want a druid that can overall play like an MTG druid on any given plane, and I can freely allow substitutions by creating and making certain druid circles available options.

I'm thinking of making the druid akin to a Summoner from Pathfinder...
Maybe Sarkhan's shifting is unique enough to only give to a Sarkhan Vol NPC, but if not, the Druid class acting as a shaman character in this case is the only one to really belong with a form-shifting spell-like-ability.

If you're doing 5e, Sarkhan could just be a dragon sorcerer, which transforms into a dragon as they level up.

Am I missing any other well-known beast-shifting characters who need accomodating in this way? Because if not, a 5e equivalent of Pathfinder's Summoners class mixed with Druid should be the best solution. Somehow though, allow for the summoning of beasts, plants, elementals, and monstrosities (since the Monster Manual handles most sea creatures as monstrosities, so Kiora needs to be thrown a bone, and some other cool native "animals" might also count as monstrosities.)

As for mana, I've *currently* decided that upon choosing a race, you'll be assigned a color.

Choose Kor, get
Choose Merfolk, get
Choose Vampire, get
Choose Goblin, get
Choose Elf, get
Choose Human, you choose.
Choose to play a Vampire on Innistrad, choose or
Choose to play a Werewolf on Innistrad, choose

At level 5, and every 5 levels after, you may choose an additional color.

Spellcasting will require either mana points or arcane points, the only reliable way to get them is to make a bond with a terrain you haven't already bonded with. If for example, a level 4 Elf PC were to bond with say, a patch of Grassland, she would gain 1 arcane point. If she bonded to a Forest, she wouod gain 1 green mana point. I would design 5 prestige classes: Alabaster Mage, Azure Mage, Onyx Mage, Crimson Mage, and Jade Mage, with their own appropriate spell lists. Using the arcane point variant system, it would take 2 arcane points to cast a level 1 spell, 3 arcane points to cast a level 2 spell, 5 for 3rd, 6 for 4th, 7 for 5th, 9 for 6th, 10 for 7th, 11 for 8th, and 13 for 9th. If said attempts to cast a spell from her spell list that is also found on the Jade Mage Spell List, spending 1 green mana point will count as having spent 2 arcane points. At level 5, the elf could choose to upgrade from to and then a portion of her arcane points would become white mana points.
The points you've accumulated SHOULD be restored after a long rest, and I might not need to, but I feel I should incorporate some method for the mana being temporary. Perhaps a system for changing arcane points into appropriately colored mana points AS you cast them?

Not 100% sure if that's how I should do it, or different terrain would be so few and far between that perhaps I would have to change it to: get +X where X is equal to.... half your level rounded down (minimum 1)/WIS for White/WIS for Green/CHA for Red/INT for Blue/Either INT or CHA for Black/roll a d3 or d4, adding the above modifier to how many arcane/mana points you gain.

Any helpful insight?

I've just about decided to leave most class features alone except changing Wild Shape to be more like summoning eidolons and summoning in general as a Summoner from Pathfinder, except suited to 5e D&D's power levels, and said summoned creatures being limited to Beasts, Plants, Elementals, and in certain cases, Monstrosities.

Following that, simply alter and add class options (like Barbarians' Primal Paths, Clerics' Domains, and Druids' Druid Circles) and allow a certain selection per plane, like Totem Armor for druids, Dragonic features to a Barbarian's rage, etc on Zendikar, and necro-alchemist-like features for a wizard on Innistrad, or more specific Domains for a Cleric of Avacyn or Patron for a Innistrad Warlock.

I believe Draconic Sorcerers can.

What is Nahiri's magic like? It doesn't have the studious nature of Wizard, Nahiri doesn't seem to be spiritual in any way, so either Druid, or Sorcerer, using the Sorcerous Origin provided for Chandra in Plane Shift: Kaladesh, but change anything fiery with anything stony.

Are they a rogue? Arcane Trickster without Spell Thief.
Are they warriory? Eldritch Knight without Weapon Bond.
Are they a tinkerer? Artificer with custom specialization.
Otherwise, Sorcerer or Wizard with custom origin or tradition.
All of them with revamped spell lists.
I guess Liliana can be a Warlock with a custom Patron and Pact Boon.

Ajani • Paladin/Maybe a "battle cleric"
Arlinn • Druid
Ashiok • Wizard/Psion
Bolas • Idk man he's a dragon with immense and varied magical capability, hard to translate that to a PC class
Chandra • textbook Sorcerer
Dack • Rogue
Daretti • Artificer (or wizard if you don't like artificers as a D&D class)
Domri • Druid
Elspeth • Paladin
Freyalise • I know nothing about her
Garruk • Hard to say. He's basically a Barbarian but he shapeshifts soooo, maybe a Totemist Barbarian? he really just doesnt have the temperament for a druid...
Gideon • Paladin
Jace • Wizard/Psion
Karn • Could be considered an artificer, otherwise I have no idea
Kaya • Rogue
Kiora • Kinda seems like a sorcerer to me actually. Water Elementalist, specifically.
Koth • Sad Boitm
Liliana • Whichever class you like being able to reanimate the dead. In d&d's base universe that'd be wizard, I suppose.
Nahiri • Yeah this one's hard. Maybe an "eldritch knight" with a focus on earth spells
Narset • Monk
Nissa • Probably a Druid, but that doesn't mean I have to like it
Nixilis • Dark Paladin or Warlock
Ral • Could easily work as a wizard or sorcerer
Ramaz • I know nothing about him
Sarkhan • Sorcerer
Sifa • Warlock
Sorin • Sorcerer/Warlock
Tamiyo • Wizard
Teferi • See Daretti
Tezzeret • Wizard
Tibalt • Sorcerer/Warlock
Ugin • creepy old man npc, except he's also a dragon
Venser • See Daretti
Vraska • Rogue
Vronos • That's a tricky. Skilled swordsman, at least somewhat of an artificer, he was an inquisitor on innistrad and those are carded as clerics... yeah I have no idea
Xenagos • Proooobably a Bard

Edit: other walkers

Saheeli: see Daretti
Samut: Rogue variant? I guess she could be a fighter, but the speedy kind
Dovin Baan: see Daretti
Sorin: Warlock, but could also work as a wizard or sorcerer