IMC much of the world was dominated by dragons a few centuries ago. Old, epic dragons whose first salvo was to slaughter the heroes of a long running war that had weakened most of the known world. A non-epic red dragon would be the minimum cap, which is CR26. It goes up from there based on how epic the dragons are.
Then you have some of the ancient things that predate dragons. The First Mourned are the very first undead and were part of a millenias-long war between certain deific beings (Life, Death, and Undeath). They are horrifically powerful and date back to the dawn of civilization. Actually the dawn of the precursor races that came from before the common humanoid races; the First Mourned think elves are whippersnappers and liked it better when the wold was Pangea. They generally stay in Twilight, a city that lies on the edge of the living realm and the afterlife, but they can be irritated into taking action.
The fey are another ancient race but IMC are not particularly involved in the affairs of mortals as they are somewhat in conflict with the current pantheon of gods. The King of the Fey is relatively young, being born after the Living-Death Wars of the First Mourned. Elder members of his court, including a redcap who may himself be the living incarnation of murder, took part in that war. The youngest fey squire, only a few hundred years old, is represented by a Leshay, CR28.
Above, or perhaps to the side, of the First Mourned are the Edicts. These are constructs that carry out some divine writ. They are infused with deific power and are almost unstoppable. A god whose portfolio covers the Edicts' writ can control them. Other gods have to exert themselves and can actually be injured by Edicts. Generally you try to distract them rather than fight them.
One, known as Oathbond, rendered judgement on an entire city in accordance with the local laws. Every broken contract, broken promise, childhood vow, etc. came home to roost. Pity the anyone who knowingly lied and said "cross my heart and hope to die" because they did, no matter how long ago.
Then there's the Screamstealer. No one's sure if it's an edict or the divine manifestation of entropy. All they know is that no magic works in its presence, no sounds can be made near it, fires grow cold, it can kill an archangel or fiendish duke with a touch, and that it leaves nothing but corpses and death in its wake. No destruction per se, just death. It was trapped last time by a group of archfiends and archangels who turned themselves into living anchors, just out of reach of the screamstealer. Good luck on getting a couple more of those guys to sacrifice themselves without a couple of gods to twist arms.
So, umm, I'm not sure what the highest non-plane-hopping CR is IMC. But I'm pretty sure I'm not going to reach it.