The Archival Worldliness Verse in the Endless Archive buffs the damage of all skills within the World category, and there are already similar sets which do the same for skills in the Class category. For instance, the Innate Axiom set grants weapon and spell damage to Class abilities, and the Claw of the Forest Wraith set grants critical chance to Class abilities. I think there should be a set like these, but for skills in the World category (which includes Soul Magic, Vampire, and Werewolf).

Here's a list of sets which require skills from a skill line to interact:

• Miscellaneous (can fit in multiple categories)
  • Shapeshifter's Chain — Reduces the cost of Transformation Ultimates (Werewolf Transformation, Blood Scion, Bone Goliath) and Werewolf abilities by 15%, and increases Maximum Health, Stamina, and Magicka by 1707 while transformed.
• Class
  • Claw of the Forest Wraith — Adds 2037 Critical Chance to Class abilities.
  • Innate Axiom — Adds 400 Weapon and Spell Damage to Class abilities.
  • Overwhelming Surge — Proc effect when dealing damage with a Class ability.
  • <Class Sets which require specific Class skill lines here>
• Armor
  • Armor Master — While you have an Armor ability slotted, increase Max Health by 5%. Using an Armor ability in combat grants you 5940 Physical and Spell Resistance for 10 seconds.
• World
  • Oblivion's Foe — Proc effect when dealing damage with a Soul Magic ability.
  • Vampire Lord — Increases the bonuses and penalties of your Vampire Stage.
  • Salvation — Reduces the cost of Werewolf Transformation by 33%. While transformed, grants 150 Weapon and Spell Damage.
• Guild
  • Spelunker — Reduces the cost of Undaunted abilities by 10%. When an ally uses your Undaunted ability synergy, restore 730 Stamina and Magicka.
• Alliance War
  • Meritorious Service — Casting a Support ability increases the Physical and Spell Resistance of up to 5 group members by 3540.
  • Powerful Assault — Casting an Assault ability increases the Weapon and Spell Damage of up to 5 group members by 307.
• Weapon
  • Red Eagle's Fury — Adds 469 Weapon and Spell Damage to Weapon abilities. Increases the cost of Weapon abilities by 5%.
  • Daring Corsair — Reduce the cost of Weapon abilities by 10%. Gain Minor Heroism after using a Weapon ability.
  • Hawk's Eye — Reduces the cost and increases the damage of Bow abilities by 6%.
  • Marksman's Crest — Increases the damage of Bow abilities against Players by 8%.
  • Wilderqueen's Arch — Bow attacks reduce the Movement Speed of any enemy hit by 15% and increase your Movement Speed by 15%.
  • Sword Dancer — Adds 600 Weapon and Spell Damage to your Dual Wield abilities.
  • Sword-Singer — Adds 600 Weapon and Spell Damage to your Two Handed abilities.
  • Light Speaker — Adds 600 Weapon and Spell Damage to your Restoration Staff abilities.
  • Spider Cultist Cowl — Adds 600 Weapon and Spell Damage to your Destruction Staff abilities.
  • Knight-Errant's Mail — Adds 450 Weapon and Spell Damage to your One Hand and Shield abilities, and restores 1537 Health when casting one.
  • Eyes of Mara — Reduces the cost of Restoration Staff abilities by 15%.
  • Wisdom of Vanus — When using a Restoration Staff, the duration of Major Mending from this set is increased with each rank of Essence Drain.
  • Robes of Destruction Mastery — When using a Destruction Staff, your Maximum Magicka is increased by 2840.
  • <Arena Weapons which require specific Weapon skills here>

I think there should be more sets like Claw of the Forest Wraith, Innate Axiom, and Armor Master, which do not require specific skill lines to proc. A set which could be applicable to all of the World skill lines (much like how the Archival Worldliness Verse functions) would make such a set useful to a variety of builds, rather than having a set be locked to use for a specific skill line (such as with the new Class sets).

Additionally, there should be more Guild sets which affect all of the Guild skill lines (like how Red Eagle's Fury affects all Weapon skill lines) and also some more Guild sets like Spelunker (which affect specific Guild skill lines). The World and Guild categories stand out as having no set which augments every skill line within, whereas you can find sets which affect all skill lines within the Class and Weapon categories.