Players would need to complete the entire campaign once before this mode would enable… so this suggestion applies to a player’s 2nd and all subsequent Alts.
I suggest a lower tier of Monoliths, perhaps starting at level 30ish. Accessible once End of Time is encountered as normal.
No need to design new bosses for it, just make it random using the existing echo types. The only “special” thing is that it would contain a large number of echoes that award either passive points or idol slots. This would then take the place of the campaign quests that award these and give an alternative levelling path.
These nodes would no longer function once the player had earned the usual max, and this would also disable the ability to earn them in the campaign so no cheaty double-dipping.
Perhaps also have a (scaled) boss encounter after sufficient Stability (or max passive and idol slots) is earned with a boss picked at random. Killing this boss would award +1 to All Attributes, replacing the final reward in the campaign.