The Next Story Boss After Viego Is Likely a Cosmic-Level Threat

First off, Viego himself was mentioned as a Celestial-level powerhouse.

Here, "Celestial" refers to a race of beings born from the cosmos, and they are inherently so powerful that mortals would call them gods.

In other words, even Viego—while not quite at the top tier like Aurelion Sol, Bard, or Zoe—is still strong enough to hang in the cosmic arena.

Just look at the lore of potential villains after Viego: Thresh absorbed Viego's power directly.

Fiddlesticks, the eldest of the ancient demons, is the strongest of them all, and Nilah's lore revealed that he has actually fought Celestials before.

In the Darkin Saga event—which isn't canon yet but has a good chance of becoming canon based on the narrative director's comments—Xolaani, in her final form, corrupted even Zoe and caused damage on a universal scale.

The Watchers are already causing cosmic-level damage through Void-spawn, and they possess a power that makes even Aurelion Sol, one of the top Celestials, tremble.

Mordekaiser has already recreated an entire dimension with his own power, and his current goal is to dominate not just the whole universe but all dimensions. There's even an official statement that he would utterly destroy Viego, a Celestial-level being.

If the rest of these guys (excluding Thresh) become the main bosses in future storylines, unlike Viego who only harmed Runeterra, they will likely extend beyond Runeterra to cause cosmic-scale damage.