The theme seems clear albeit a bit narrow perhaps.

I would dump up those abilities/stats:

>
> [+2]: Put a mist counter on each of up to two target nonland permanents. (If a permanent with a mist counter would become tapped, remove a mist counter from it instead.)
> [0]: Remove any number of mist counters from among permanents you control. Draw that many cards.
> [-7]: You get an emblem with "Permanents you control have hexproof and spells you control can't be countered."
[5]

I might be tempted to change that second ability to being able to remove any kind of counters. Maybe up to four in total though so it isn't so busted with cards like Dark Depths. There probably would still be bonkers stuff you could do with it though.

Well, the ultimate is pretty much "Win the game" against anything except a creature-smash deck. The tension between the first two abilities and the more obvious use of the first ability as a lockdown is odd.

Ah; problem. nothing stops you just piling the mist counters onto an enchantment or an artifact that won't ever tap (or care if it does) anyway.

I find it a pity that Sorah's first ability will never interact with her second ability without outside help. I mean, I presume other things make mist counters in this set. But I'm not sure I like the idea of a Planeswalker that's beholden to the other cards in the set...

Maybe the second ability could be "Remove any number of counters" instead of identifying mist... oooh, wait, it isn't from a SINGLE permanent you control. You could remove all your -1 counters; which is a fine planeswalker ability, but not at -1.

So, couple things about this card....

My intent for Mist Counters (which I’m fairly certain I articulated incorrectly in their rules text, based on some of the comments) is that they are beneficial, representing an invigorating, rejuvenating cloud. A Mist counter is supposed to be used to “pay” for tapping. For example: instead of tapping a creature in order to attack or as a cost for an activated ability, the mist counter is removed instead, like a one-shot version of vigilance or an inside-out exhert. It would also protect against the typical blue “tap target thing” play. I think Tahazzar’s wording is correct? I’m not sure. The +1 can also be used to grant mist counters to permanents you don’t control, which would mostly be for politics/multiplayer games.

The -1 should probably be limited at 4, though since my intent is that only Sorah planeswalkers will create mist counters, drawing tons of cards will be a 3-4 turn investment.

As for the ult, i’d like to leave the emblem as-is, though I’m not sure if -6 or -7 is appropriate for simply granting hexproof.

> [+1]: Untap up to one target nonland permanent. Simple.

In a similar vein.

> (Whenever the permanent taps to attack or activate an ability, you may remove a mist counter from it to untap it.)

Makes it obvious that only the untapping is undone and not the effect paid for.

I see little reason not to allow any untapping to be done though. Why restrict it to attacking and activating an ability?

Yeah; the first is just untapping a thing. (It's slightly better, since you can leave it and it then sorta becomes vigilance)

Limiting the most of those you can consume seems like a brute-force safety; but it might be needed. But when would you ever use that ability. The ultimate is so much stronger.

It's also weird to have a planeswalker whose middle ability is not usable when you cast it.