ZOS is very clearly NOT going to heavily differentiate PvE and PvP because it is their belief that once you "learn the mechanics of PvE that works the same in PvP so you don't have to relearn a whole new game". I agree with this.

The core idea is for PvE and PvP to:
-feel like the same game [essentially be cohesive and not feel like you're stepping out into another different segment]
-use the same mechanics and systems [incl. sustaining]
-have shared character progress [things you get in PvE you can use in PvP and viceversa, so no matter what you do you are still benefiting your character progress]

This is GOOD.
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However, this utopic dream cannot be achieved without SOME differential treatment, once you start to take into account the different and unique factors of each gameplay context [PvE & PvP]. PvP having intelligent and adaptive enemies as opposed to PvE, as well as more heavily leaning towards min-maxing builds & providing less choice. This is inherent to a PvP system and can only be mitigated.

Attempting to balance them both with ABSOLUTELY NOTHING to account for these inherent differences- dictated by the nature of the enemies you are facing- is a futile task. Because you MUST account for these factors when doing anything game-balancing, and they're big factors, too. Currently ZOS is trying to balance these two ignoring the fact that they are very different mediums.

This is BAD.
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With that being said, a closer and happier balance between PvE and PvP can be achieved by shifting their approach and "compromising", for the sake of universal game health.
It's all in the numbers. If it's out of the question to change CP/gear/abilities [the latter of which is strongly negative] then perhaps Cyrodiil as a zone should get its own "scaling".

My suggestion resides in the idea of adding unique PvP-only scalings to players in Cyrodiil & Battlegrounds. For example- in PvP everybody has -5% Resource Regen [if that becomes an issue].

Everything displays the same to the player and the numbers and relations between them stay the same, relative to each other. You have 3k spell damage? You keep 3k spell damage. 25% Magicka Regen from CPs? It stays. But all that stuff is scaled, for everybody, down or up or sideways, that's ZOS' analysis to make and decide.

One issue crops up in PvP, no problem, adjust the PvP scalings in that area. Too much sustain in PvP? Scale it down in Cyrodiil/Battlegrounds, while keeping it intact in PvE. Too much damage in PvP? Scale that down, PvE gets to keep it, so you don't have to nerf everybody [then also nerf the PvE content because it's kicking our freshly nerfed asses... or worse, ignore the corpses littering trials floors.]
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The most popular suggestion is different CPs for PvE and PvP. However, as with any solution or adjustment to this, to be mindful of the goals, they must be checked against them, and this is sooort of slightly perhaps too much eating into the cohesion. Personally I believe it sounds like a great idea, but ZOS seems to disagree.

My suggestion might be silly or have unforeseen issues, but after all this is a discussion, and perhaps other individuals might have better ideas on how to preserve the goals of cohesion between PvE and PvP while also maintaining healthy ecosystems in both cases.