The Fiend Folio of D&D introduced us to a class of creatures called the inevitable.
While drastically under-powered compared to their flavor text in the Fiend Folio, in mutants and masterminds they can represent a real force to help curb some of the broken character concepts, without outright banning them.
Inevitables are Lawful Neutral creatures from the plane of Mechanus, whose job it is to enforce specific "natural" laws. While generally clockwork in appearance, they are not golems, and they can grow personalities (though these are purged when they return home).
In many ways, they act as DEATH from the discworld novels, or the Time Wraiths from the Flash TV series. Inevitables can hunt a PC who is using a broken concept too much, or "glitching" the game with cheesy manuvers.
The Inevitable of Oaths- This would be the weaker of the inevitables, but it seeks to punish oathbreakers and to force them to keep their vows. Armed with compulsion effects (as well as combat ability) they seek to outright MAKE a PC who has broken solemn oaths make good on their word. While regular lying is too commonplace to summon such a creature, a hero who promises a villain succor, and instead kills them might bring one, or one who is guilty of treason, killing a guest in his home, or other such atrocities.
The Inevitable of Time- For the PC who breaks time travel, a Time Inevitable can appear- not to fix a paradox, mind you, but to punish the super who caused them. (Though it might seek to repair a damaged timeline). Characters who use very liberal ideas of time travel to duplicate objects by keeping versions of the same object from different timelines or times is a great example. These creatures might keep a PC suspended in stop time eternally, or use various temporal abilities (precog, slow, haste, etc) to combat them and destroy them for their infractions.
The Inevitable of Death- While resurrection and regeneration are common in comics and in the game, as are immortals immune to aging, the Death Inevitable might come when the books are drastically out of balance. Perhaps he kills someone else for each person resurrected, or he comes to punish a PC who resurrects every dead body in the cemetery, or who solves every crime not by finding the killer, but just resurrecting the victims. This inevitable might take on a "shape" much like the "antagonist" of the Final Destination movies, circling back to reclaim its victims or take new ones. They might also seek out characters who transform themselves into powerful intelligent undead, such as liches, or who harvest souls and prevent them from moving on.
The Inevitable of Divinity- While there may be gods, there's only so much faith to go around. Since faith sustains gods, if too many people were gods, all gods would be too weak to be very impressive, and their low faith rations would shrink even more. Thus the Inevitable of Divinity seeks to limit the number of gods. Perhaps he challenges those who would ascend to Godhood, or attempts to slaughter priests who create new cults. Perhaps he seeks out gods who have lost too many followers to "make room" for the other gods. While the in-game applications of this inevitable might be limited, it might make the characters think twice about getting "all the powers" and becoming a full god themselves.
Keep in mind, inevitables are not meant to end campaigns. Rather, they are intended to be creatures that come forward to punish characters who abuse natural laws and create conflicts for a PC to overcome, and a new mission strand. PCs should be able to avoid or defeat an inevitable, but know that they might summon more if they continue to abuse game balance destructive powers, and it might encourage them to limit their uses (or at least not to cheese monkey time travel).
If nothing else, the inevitables might be after the NPCs, giving the PCs something to do.
