TL;DR: Altmer's off-stat sustain is compromising Dunmer's unique gameplay style as the hybrid race. This is costing each of these races their defining racial identity, for if you've played as an Altmer you might as well have played as a Magicka specced Dunmer also. To differentiate the two races a little better, Altmer's off-stat sustain should be converted to a slight boost to magicka use in some way that DOES NOT directly affect their raw damage potential or magicka sustain in the damage dealer role and Dunmer should inherit an off-stat sustain passive (at the cost of a minimal amount of max resources ONLY IF NECESSARY). Proposed changes (in bold):

Altmer
-Increases Experience gain in Destruction Staff Skill Line by 15%, extra 1% Generic Experience gain
-Increases your chances of applying status effects by [x]%***. Reduces damage taken by 5% while you are using an ability with a cast or channel time
-Increases your Max Magicka by 2000
-Increases your Spell Damage by 258

*** Here is a list of possible alternatives to this passive that have been suggested in the comments:
-Increases the amount of resources restored by fully charged heavy attacks by [x]. (This passive would synergize particularly well with their current 5% damage reduction while casting, as heavy attacks are considered channeled/cast time abilities)
-Restore [x] Magicka when you use a healing ability. This can occur every [y] seconds.

Dunmer
-Increases Experience gain in Dual Wield Skill Line by 15%, reduces damage taken from Lava by 50%
-Increases your Max Stamina and Magicka by 1500-1750 (Down from 1875 ONLY IF NECESSARY).
-Restore [y] Magicka or Stamina, based on whichever is LOWER, when you deal direct damage. This effect can occur once every [z] seconds. Increase Flame Resistance by 2310. You are immune to the Burning status effect
-Increases your Weapon and Spell Damage by 258

The following is an analytical comparison between both of these races and detailed justification for the proposed changes:

Greetings. As most of you already know, ZOS is currently trying to rebalance racial passives to more equalize the races whilst also making the races more accommodating to every role (tank, DPS, or healer) and retaining each race's unique feel and gameplay. For the most part, they've done a pretty good job. I feel most of the races are pretty balanced, unique, and also viable for every role. One area where they fall short, in my opinion, is giving the Altmer and Dunmer unique racial identities. Here are their current racial passives on the PTS:

Altmer
-Increases Experience gain in Destruction Staff Skill Line by 15%, extra 1% Generic Experience gain
-Restore 645 Magicka or Stamina, based on whichever is lower, after activating a Class Ability. This effect can occur once every 6 seconds. Reduces damage taken by 5% while you are using an ability with a cast or channel time
-Increases your Max Magicka by 2000
-Increases your Spell Damage by 258

Dunmer
-Increases Experience gain in Dual Wield Skill Line by 15%, reduces damage taken from Lava by 50%
-Increases your Max Stamina and Magicka by 1875
-Increases Flame Resistance by 2310. You are immune to the Burning status effect
-Increases your Weapon and Spell Damage by 258

ZOS has stated their attempts to balance the races are based off of end game content. As such, we will analyze these races accordingly. In most endgame content, offensive power (max stats, damage, and sustain) is what most would agree hold the greatest importance. It is for this reason we will primarily focus on these stats and for the most part ignore the Altmer's damage reduction while casting and the Dunmer's fire resistance (as these defensive bonuses could conceivably cancel each other out from a racial balancing perspective anyway).

As we can see, Altmers possess great bonuses to Magicka offensive power, and are quite intended for endgame Magicka builds. Dunmers, on the other hand, possess equal bonuses to both Magicka and Stamina offensive power. However, because hybrid builds (builds that use both offensive abilities scaling off of Stamina and offensive abilities scaling off of Magicka) are not viable for end game content (due to splitting of resources, lack of synergistic CPs, etc...), we will examine Dunmers as either endgame Magicka OR endgame Stamina builds. To more directly compare the Dunmer against the Altmer, we will focus on their Magicka build. With this in mind, here are their offensive stats:

Altmer Offensive Passives
-2000 Max Magicka
-258 Spell Damage

Dunmer Offensive Passives
-1875 Max Magicka
-258 Spell Damage

Right off the bat we can see their similarities. In fact, outside of the additional 125 Max Magicka the Altmer get, they are identical. But what about their off-stat passives?

Altmer Secondary Stat Passives
-Restore 645 Stamina after activating a class ability every 6 seconds

Dunmer Secondary Stat Passives
-1875 Max Stamina
-258 Weapon Damage (though this can be largely ignored, due to the vast majority of endgame Magicka builds synergizing off of Spell Damage and not Weapon Damage. A notable exception would be dual wield cast builds, however this passive even in those builds would only affect light attack damage, and not ability damage.)

So even in their off-stat passives, the two races seem pretty similar. They both provide very limited increased Stamina utility (for roll dodging, blocking, breaking free, etc...). Altmers get it through a little bit of added stamina sustain, while Dunmers get it through a bit more Stamina to begin with (larger max stamina). So even after taking these off-stat passives into account, the two races appear to be (in almost the most literal sense of the word) perfectly balanced. They are both great Magicka builds (the difference being only a measly 125(!) Max Magicka) and each provide a bit of Stamina utility via either sustain for Altmers or max resource pool for Dunmers. So why play an Altmer over a Dunmer? Why a Dunmer over an Altmer? They are virtually identical. Where is the racial identity?

Lore dictates that Altmer are among the most powerful and distinguished spell casters in the realm. Dunmer, on the other hand, are extremely nimble and athletic warriors that are also proficient in destruction magic. Why then, are they almost indistinguishable from one another in endgame content? Why don't their racial passives reflect these differences? As the racial passives currently are on the PTS, neither race is adding a unique flavor to the game and they are both overshadowed by each other.

So what can be done?

Altmer passives should play into their dominance as mages found within TES lore. I believe granting them a passive to reinforce their use as magicka users without directly increasing their raw magicka power or magicka sustain would help establish their racial identity. An off-stat sustain ability has no place on the Altmer's racial passives. Instead, it belongs to the Dunmer, the race known for their spell casting and athleticism. I would like to see this added to their passives, even at the cost of a minimal (very minimal, as Dunmer are currently in a good spot for magicka DPS) amount of max resources if need be to maintain balance. These changes would help distinguish Altmer as superior mages to the Dunmer (via a small boost to magicka usage and a slightly greater disparity between max resources) and also keep them in the running for top Magicka DPS parses without pushing them far ahead of the competition (as on PTS they are currently in a very good spot). These changes would also showcase the duality of the Dunmer's nature as both skilled warriors and as proficient spell casters, by providing unquestionably the most off-stat utility out of any race via both sustain and max resource pool. With a Dunmer specced into Magicka, this would provide greater ability to sprint, roll dodge, break free, bash, and block effectively, while still putting out very competitive endgame Magicka DPS.

Here are my proposed changes (in bold):

Altmer
-Increases Experience gain in Destruction Staff Skill Line by 15%, extra 1% Generic Experience gain
-Increases your chances of applying status effects by [x]%***. Reduces damage taken by 5% while you are using an ability with a cast or channel time
-Increases your Max Magicka by 2000
-Increases your Spell Damage by 258

*** Here is a list of possible alternatives to this passive that have been suggested in the comments:
-Increases the amount of resources restored by fully charged heavy attacks by [x]. (This passive would synergize particularly well with their current 5% damage reduction while casting, as heavy attacks are considered channeled/cast time abilities)
-Restore [x] Magicka when you use a healing ability. This can occur every [y] seconds.

Dunmer
-Increases Experience gain in Dual Wield Skill Line by 15%, reduces damage taken from Lava by 50%
-Increases your Max Stamina and Magicka by 1500-1750 (Down from 1875).
-Restore [y] Magicka or Stamina, based on whichever is LOWER, when you deal direct damage. This effect can occur once every [z] seconds. Increases Flame Resistance by 2310. You are immune to the Burning status effect
-Increases your Weapon and Spell Damage by 258

In short, these changes would differentiate these two races enough from one another that playing each would give gameplay a unique and flavored feel that currently isn't there. It would also give more options to consider for endgame builds. Players would have to ask themselves "do I want a truly, nails to teeth min/maxed build for Magicka DPS that excels in very controlled endgame content like trials? Or do I want a more versatile and defensive playstyle that excels in worst-case-scenario situations like Veteran Maelstrom Arena and PvP?" If squeezing out the last, tiny bit of DPS is a goal, Altmer should be chosen. If a player desires more versatility, Dunmer is the race to be. Both would be equally competitive in endgame content, though they would each excel at different things.

I know this analysis was focused on Dunmer vs. Altmer as Magicka builds, but a similar situation is present on the Stamina side of things between Dunmer and Orcs, albeit to a lesser extent thanks to the Orc's unique passives that keep them alive longer in fights. The same aforementioned changes to the Dunmer, however, would also help to even further differentiate them from the Orcs by providing Stamina Dunmers greater access to Magicka utillity for things like crowd control, self heals, and various other survival mechanics that rely on magicka usage (Nightblade cloak, looking at you). This too would help reinforce the Dunmer's identity as a hybrid/jack-of-all-trades race and support their lore as warrior-casters.

I've posted a link to this thread in a comment on the official racial change feedback thread (link found below) and if you guys support these changes please do the same. The key is to make enough buzz that it gathers the attention of the development team, so tagging a dev in your comment on that thread couldn't hurt either. Please let me know what you guys think and comment your thoughts, critiques, and objections, as the best suggestions can overcome adversity.

Link to official racial change feedback thread: https://forums.elderscrollsonline.com/en/discussion/454514/pts-update-21-feedback-thread-for-racial-passive-changes#latest

P.S. I've posted about this issue before, but finally took the time out to thoroughly and completely explain my reasoning. I apologize for the wall of text and any perceived spam.

Edit: Implemented into the original post some ideas discussed in the comments

Edit2: Changed values of proposed Dunmer resource pools, as the initial proposition of 1500 was too low (debated in comments below)

Edit3: Edited to reflect my current preference of the Altmer's passive to be increased chance to apply status effects.

Edit4: Added additional clarity to the stances I take in the TL;DR section and moved it to the top of the thread for easier access.