The Chthonic Lands can be heated or cooled with engineering tricks. This allows tropical or arctic critters to expand their ranges greatly, occasionally resulting in them appearing on the surface in places they normally avoid or can't reach.
Looking at the dungeon encounter tables in the 1e DMG I noticed something odd. How do the deeper level monsters travel from dungeon to dungeon without passing through the upper levels? Is there some hidden method that monsters have that is completely unknown to humans and their allies?
Most humanoids and some other monsters have doorkeepers, individuals that can command entrances to open or close. The attunement process takes time and effort and one entrance can have multiple doorkeepers.
Primates can convince trees to act as rafts and stones as tools. As long as their relationship is positive, the plants and stones reshape themselves to the needs of their allies.
Spelljammer monitors (second MC) are police for Wildspace. What isn't well known is each has a base of operation that is staffed by base modrons. Their need to spread Law throughout the spheres has brought both races together and so far their work as spoken for itself.
One moat enchantment turns some of the water into mist, enough to conceal the entire castle. Used in combination with Creation spells to create traps and opposing forces had better have a wizard on hand lest they lose most of their forces before spilling any blood of the castle dwellers.
Ocean is grandfather to all. His mercurial moods were inherited and the cause of most conflicts.
One mountain chain is a battlefield. Each peak wants to be the highest and they all use erosion causing weather to wear each other down.
Bounty hunters that prefer to take targets alive may invest in one use melee and thrown weapons that deform and engulf one of the target's limbs.
An alien invasion by beings that weave light and stone together are starting to deplete the sun and world. Those lucky enough to be able to leave the surface may find that the new world made partially of light may be worth colonizing.
Soul-less bodies of all sorts of creatures fall from rifts in the sky. No one knows where they come from or why, though scavengers and necromancers congregate and fight over choice corpses.
An arcane warrior spell that turns bones into functional weapons is handy in situations where metal is undesirable.
Some wizards and mad scientists hide their lairs in the wilderness, other planes/planets, or in plain sight to avoid the annoying adventurers that want to kill them and loot their homes. A few use mobile homes, usually with some form of teleportation, as this allows easier escapes, better retention of personal wealth and equipment and the ability to go to places where raw materials are to be found for their experiments.
Being an apprentice to a wizard is a hard thing. The worst is that they are infused with some of their master's personality traits. There is no way to avoid this during training and most don't bother trying to reverse it afterwards as there are more important things, like accumulating power.
Nixie slaves do not suffer the effects of aging so some elderly willingly travel to their waters and offer themselves for eternity.
Harpies may have songs that only affect those under the effects of magic (or any other limitation you can think of).
One advanced psionic culture has enslaved illithids and use them to produce the psionically active mucus that has so many applications in psychic industry. Criminals are used for metamorphosis and food. As there is no resulting elder brain, there is no illithid culture or much of a resistance. (Inspired by Alien comics.)