--- TL;DR ---
For certain core sets in the game ensure there are balanced sets for magicka, stamina, and tanks when feasible. Specific core areas to address:
1) High damage: No change recommended...these sets seem mostly balanced to me. Julianos balances Hundings, and there are tank sets that bring various damage options to the table.
2)
Penetration: Balance is needed. Stamina users have access to Night Mothers Gaze, but there is no equivalent magicka option that can be crafted.
3)
Resource management: Balance is needed. Magicka users have access to the Seducer set, but there is no equivalent stamina option that can be crafted.
4)
Stat pools: Balance is needed. Update 12 will bring us the medium Draugr Hulk set that provides just about 6000 stamina, an equivalent heavy set (Plague Doctor) provides about 8000 health, but we have no light armor set that provides 6000 magicka.
5)
Buff sets: Balance is needed. Tanks have access to sets that provide Major Brutality (Dreugh King Slayer), Major Sorcery (Update 12, Rattlecage set), and Minor Force (Update 12, Medusa). These are beneficial buffs that would also benefit magicka and stamina play styles. Something unique about buff sets is that they give you more flexibility about what skills you slot.
6) Jewelry sets: No change recommended. Stamina, magicka, and tanks have the Agility, Willpower, and Endurance sets as good starting points when considering jewelry options.
If the specific areas above were addressed it would help achieve set balance, and support a broader array of play styles.
I'm not saying we need 3 mirroring sets that satisfy stamina/magicka/tank players for every set in the game...there are niche sets for specific play styles and I think that is cool. But core sets that provide key values should be watched closely and balanced when possible. Monster Sets are unique and I don't think we need 3 flavors of each set to satisfy stamina/magicka/tanks.
--- end TL;DR ---
With Update 12 just around the corner and a whole slew of tantalizing sets wrapped in that package, I think now would be a great time to rectify some set imbalances between the various Light, Medium, and Heavy armor sets. I think this is especially true because there were set imbalances before Update 12, and based on the new sets here in the PTS the imbalance continues. A few examples follow to highlight my concerns (these examples are based on the Update 12 sets).
--- EXAMPLE 1 ---
The medium Draugr Hulk set provides just about 6000 stamina, an equivalent heavy set (Plague Doctor) provides about 8000 health, but we have no light armor set that provides 6000 magicka. One could argue the already existing light Necropotence set is the equivalent of the Draugr Hulk set, but I disagree, as the Necropotence set only provides similar magicka stats if you have pets active which is a limitation not imposed by the Draugr Hulk and Plague Doctor sets.
--- EXAMPLE 2 ---
The heavy Medusa set provides magicka set bonuses and a 5pc Minor Force buff (crit damage increased by 12%). Where is the medium armor equivalent that has stamina set bonuses with the same 5pc bonus? And the light armor equivalent that has solely magicka stats (i.e. no health bonus) and the same 5pc bonus?
--- EXAMPLE 3 ---
The heavy Rattlecage set provides magicka set bonuses and a 5pc Major Sorcery buff (20% spell damage). However there is no medium armor equivalent that provides stamina set bonuses and a 5pc Major Brutality buff (sorry, Dreugh King is a heavy set and doesn't count as a pure stamina set) . And there is no light armor equivalent that has solely magicka stats (i.e. no health bonus) and the 5pc Major Sorcery buff.
The imbalances between sets precedes Update 12. Consider that for a long time stamina players had the crafted Hundings Rage set, but the equivalent crafted Julianos set for magicka players wasn't introduced until around October 2015 with the Orsinium DLC. Another example...currently stamina players have the Night Mothers Gaze set to gain extra physical penetration, but there is no crafted equivalent for magicika users that provides spell penetration. Another example...the crafted Seducer set for magicka players provides good magicka recovery and magicka cost reduction (8%)...but there is no stamina equivalent to the Seducer set.
Some might counter that balancing the sets in this way would lead to OP combinations. For example, if a crafted stamina equivalent of the Seducer set was added (we'll call it StamSeducer for now)...then stamina players might run around with a 5pc StamSeducer and 5pc Vicious Ophidian set and thereby achieve 16% stamina cost reduction. I agree that some interesting and potentially powerful combinations would be created by striving to balance the key sets.
However core set balance should be the starting point and the foundation first...then minor tweaks can be made if OP combinations emerge.
Some might counter that having core sets balanced would be "boring" or lead to situations in which everyone is using different flavors of the same set for magicka/stamina/tanks. I disagree. Consider just the existing stamina sets for a moment. In general we know that for stamina users Hundings Rage, Night Mothers's Gaze, and Agility set combos are common choices...but all stamina users don't run just those sets. From a PvP perspective, we also see combos like Viper and Velidreth (burst), Clever Alchemist and Molag Kena (burst), Vicious Ophidian and Marksman (sustain), Black Rose and Vicious Ophidian (sustained damage/defense), etc. My point is that even just for stamina players there are a lot of different approaches to take that can be customized through your CPs and your sets. More damage or more penetration? More burst or more sustain? Group play or solo play? Ganking or dueling? Ranged or melee? These choices lead to a variety of play styles, which can rely on different sets to maximize their potential. If one is to really argue against core sets being balanced...then take away Hundings but keep Julianos. Take away 1 of the Agility, Endurance, or Willpower sets. Change the Night Mother's Gaze set so that it's magicka based and only provides spell penetration. But that wouldn't feel right, correct? And that's my point...
we already have some balance between the sets...just extend this balance to some more key sets.
Overall I think really striving to achieve set balance supports a broader range of play styles and possibilities in how we gear up for battle, whether in PvP or PvE. Actually we can see ZOS take this balanced approach in the Update 12 sets. For example, the light Spinner's Garments set provides spell penetration...and the equivalent medium Spriggan's Thorns set provides a similar amount of physical penetration. So I applaud ZOS's efforts to balance the sets, and I say take it a step further and balance some of the other sets I've mentioned in this discussion.
What do you ladies and gents think?
*EDIT to clarify a few points in bold*
*EDIT to add a TL;DR. Also want to say thanks to
@potirondb16_ESO and
@Mr_Nobody for their insightful feedback and helping me to further refine my stance.*