As shown by the late-game research of Thaumic Augmentation, the player seems ready to fight the Eldritch. This could be a way to make that happen.

There could be some kind of ritual to taunt the Eldritch, luring them into the world to fight the player. The ritual would end up summoning an Eldritch Viceroy, an Eldritch "representative." They would have a name resembling "Viceroy _____ of _____", the first blank being some dramatic name derived from Latin words, and the second blank being a kind of title. This title would determine the fighting style of the Viceroy. A few examples would be the use of certain Foci against the player, or being able to summon and control taint at will. They would likely also make use of Runic Shields.

They would likely wear a dark cloak, similar to the Void Thaumaturge Robes, with a special crest relevant to their title. They could use a Caster's Gauntlet against the player, or some other kind of Eldritch magic similar to that used by Eldritch Guardians. They could also make use of a Primal Crusher or Cutter.

They could drop items that expand upon the player's understanding of the Eldritch, and lead to research unlocking the dark magic used by Eldritch Guardians. They could also drop curios that can be used for further research (similar to the Crimson Rites).

The death of a Viceroy would infuriate the Eldritch. This could lead to a few interesting things, like some kind of "favor" system, where the player could either do things to help or hinder the Eldritch. It could be something similar to Warp, but separate.

There could also be a few weaker Eldritch, called Eldritch Servants. They could be similar to the Crimson Cultists, but using Eldritch magic. They could spawn out of Eldritch portals that appear when the player has too low favor with the Eldritch.

A further concept could be bosses spawned through favor, that could drop items that remove effects of low favor, letting the player infuriate and combat the Eldritch without side effects. This could also lead to some kind of player "redemption" for all that warp the player gained earlier.