A few ideas

Grenade110

As I know you have made older planeswalker for the game aka Tezzeret and Sarkhan just to name a couple but I've also noticed that white is the only color to not have a monocolor that is not one of the original five or a 2nd version of the five.

White has Gideon1 and Gideon2
Blue has Jace1 and 2 and also Tez
Black has Lily1, 2, and Ob Nixilis
Red has Chandra1, 2, and Koth
Green has Nissa1, 2, and Garruk

This bothers me because I have never liked Gideon too much on cardboard and I'm not a fan of Gideon2 in PQ.

Of course options are limited to Elspeth as she is the only other strictly monowhite walker nowadays. Yes, Ajani has white versions but he has been in WG and WR versions.

My idea for Elspeth's abilities stay with Elspeth, Sun's Champion.

1: Create 1 1/1 Soldier, upgrade like Garruk to 2 then 3 then 4.

2: Destroy a creature with power 6+, lower power required to 4+, then make it all with 4+, at stage 4 drop the loyalty cost 2 or 3.

3: Create Support give your creatures flying, stage 2 give a +1/+1 To them. Stage 3 +2/+2 And finally +3/+3 And flying.

I'm not sure what would be right loyalty levels.

Another idea is to alleviate the burn of getting duplicates at higher rarities. Commons are basically fine but maybe upping them to where uncommons are now would be nice. As for Uncommons, Rares, and Mythics have a system in place where you can convert them once for another card of the same set and rarity at random once and if the replacement card is also a duplicate it will convert to Crystals at a rate of 10 for uncommon, 20 for a rare, and 50 for a mythic.

I am a bit OCD and I have this urge to have two walkers of each color, and one of each color combination. I've noticed that some color combinations are not yet represented. And the options for other is less than stellar to me. I have a strong distaste for Arlinn.

Look at the 2 color walkers available:

White-Blue: Dovin Baan
White-Black: Sorin
White-Red: Ajani and Nahiri
White-Green: n/a
Blue-Black: n/a
Blue-Red: Saheeli
Blue-Green: Kiora
Black-Red: Sarkhan
Black-Green: n/a
Red-Green: Arlinn

I would like to see Ajani fill in the WG spot, Ashiok in UB, and we'll either the cursed version of Garruk which would be awesome or Vraska in BG, and I'd love to see Domri Rade or Xenagos make an appearance in RG.

Ajani, Valiant Protector would be a great addition.

Ability 1: Give a creature +1/+1. Upgrade to +2/+2 And +3/+3, then +4/+4.

Ability 2: fetch next creature. Then have it gain mana at higher renditions. Stage 2 it could gain 2, 4 at stage 3, and 7 at stage 4.

Ability 3: reinforce a creature. Stage 2, reinforce it and gain some life, stage 3 add trample in the mix for that turn. Then stage 4 give it berserker and prevent damage for that turn.

Ashiok would be trickier but here is my idea

Ability 1: Opponent discards a creature card. Stage 2, they discard creature and you get a 1/1 Nightmare, stage 3 just double the nightmare and at 4 it be 3 1/1 nightmares. Still with the discard effect.

Ability 2: Give a creature +2/+2 And it becomes a nightmare. Stage 2 slap deathtouch on it. Stage 3 berserker and an extra boost making it +3/+3. Stage 4 a final boost so it's +4/+4 And maybe give it flying.

Ability 3: Opponent discards hand. Stage 2 You get 1 1/1 nightmare. Stage 3 make it 2 1/1s and at stage 4 it can be 2 1/1 nightmares per card discarded.

As for Garruk go with Apex Predator.

Ability 1: 1/1 beast, stage 2: 1/1 beast with deathtouch, stage 3 2 1/1 deathtouch beasts, and finally stage 4 gives 3 of them.

Ability 2: Destroy a creature. Then at the higher stages add life gain either in increments of 3 or 5.

Ability 3: Create Support giving your first creature+2/+2 And trample until end of your turn when it attacks. Stage 2 it hits all your creatures, Stage 3 double the power/toughness boost, stage 4 it becomes +7/+7.

Of course these are just my ideas but I think they would be doable and interesting.