First of all, I would ask fellow forum members not to blow the changes out of proportions. Unless devs changes the racial passives a lot from what they are on PTS right now, it wouldn't not affect PvE DD performance significantly. However, still I wanted to theorize what the effects might be on current PTS balance.

ZOS has expressed their intent to convert all three races Orsimer(S), Dunmer(S&M) and Altmer(M) into glass cannons of their respective categories. Simplification below.

Orsimer

- Max HP
- Conditional Stamina Return
++ Max Stamina
+ Conditional Health Return(Possibly a buff to current one on PTS)

Note - This is a buff for PvE DPS, if the Stamina bonus is greater than 1000 (Similar to current Dunmer on PTS, who outperform Orc by a slight margin). The sustain loss can be mitigated by other means, especially in classes with higher sustain and won't be felt much, if not using a Weapon spammable skill.

Dunmer

- Max HP
+ Max Stamina
+ Max Magicka

Note - This is buff for PvE DPS overall, no matter what the changes are.

Altmer

- Conditional Magicka Return
+ Utility

Note - This is most probably a nerf for PvE DPS overall. Might a good buff for PvP, depending on what this utility is.


Potential Issue

The primary issue comes with balancing the potential values of the stat bonuses of Dunmer with the other two, as all three races will become glass cannons. Simply speaking, the role each race will have in the greater picture of DD balance for PvE and PvP is fairly clear, apart from Dunmer.

Altmer - Magicka Static Glass Cannon
Breton - Magicka Unconditional Sustain
Orsimer - Stamina Static Glass Cannon
Redguard - Stamina Conditional Sustain
Bosmer - Stamina Unconditional Sustain
Khajiit - Dual Stat Dynamic Glass Cannon

and finally

Dunmer - Dual Stat Static Glass Cannon

And here is where the issue begins. Dunmer is competing with three different races for its niche in overall balance.

Altmer vs Dunmer - Both races are glass cannons. Essentially for Magicka PvE, for them to be equals, Dunmer will need 2000 Magicka. However, given ZOS's intents to keep the magicka and stamina bonus same, this will make Dunmer OP in the Magicka PvP department and as well, in the Stamina PvE department. No matter how good the Altmer utility is for PvP, 2000 Stamina+Flame Resist+Burning immunity will better for Magicka PvP for utility, especially for Vampires.

Orsimer vs Dunmer - Again both races are glass cannons. Now that Orsimer is a true stamina glass cannon, there is no reason for them to get beat by Dunmer in stamina raw damage. For that to happen, they will need equal or more stamina than Dunmer. If Dunmer get 2000 stamina due to the other issue, then Orcs will probably get that as well. This opens a different can of worms, that being, 2000 stamina+254 buffable Weapon Damage will shoot their stamina DPS significantly higher than all other stamina races as both already outperform the rest by a slight margin on PTS, especially on raw damage classes.

Khajiit vs Dunmer - This will be the third issue. Currently on PTS, Khajiit gets higher highs than Dunmer and lower lows than Dunmer in PvE DPS, resulting in slightly higher magicka DPS averages on crit classes, slightly lesser magicka DPS averages on non-crit classes and slightly lesser stamina DPS averages on stamina classes owing to their Dynamic Glass Cannon nature. If Dunmer gets more both of damage stats, Dunmer will get the same max parses as Khajiits but significantly better lowest parses, resulting in higher DPS averages across all classes, except Magicka Templar. And this is just PvE. For PvP, Dunmer passives blows Khajiit passives already out of the water, Weapon/Spell Damage is arguably much better a stat bonus than Weapon/Stat Crit Chance.

And even if devs somehow manage to equate Dunmer with the rest of the races, they will still be technically the best glass cannon for types of game play (Magicka/Stamina PvE/PvP). If they intentionally make Dunmer weaker or 2nd/3rd best for each individual aspect of DD, while still good for them all, there will be people complaining how Dunmer lost its value, because it is not the best in anything. From what I am seeing from a theorycrafting perspective, there will be no way appease every one for the next changes.

Thanks for reading. Keep in mind, this is all speculation. I will do proper tests once the changes are up and update with actual result data in a few days.