Plane of Fire

One of the most commonly worshiped planes due to its many natural occurrences and its relative inoffensiveness: While drawing from it can be destructive, it does not unleash eldritch horrors and create abominations in the same way that the Plane of Darkness tends to. Deserts tend to have a naturally weak boundary to the plane. Creatures in nature draw from it, often to create weapons for themselves (e.g. salamander fire breath).
The plane itself is essentially the inside of a sun, constantly searing heat and pressure and burning gases. Some cultures believe that the plane of fire is the sun, just brought closer. Overall, the Plane of Fire is a boring place from a human perspective, with most unable to truly conceptualize or find any meaning in the temperature (and therefore pressure) differentials that make up its 'geography'.

Impact on the world

Connection to Fire is a key feature in the biology of arcahe and salamanders. The former use it to generate a pressure differential necessary for the function of their hydraulic limbs, and the latter use it to... breathe fire.
Glasswyrms are fire-based spirits found in the mundane world. Ignium, key to the technological revolutions of the Terian Empire, is formed as a result of these creatures.
The Molten Desert is a region experiencing a relatively slow thermal runaway, slowly melting into a sea of molten glass, as a result of the feedback loop between increased temperature and increased affinity to Fire.
The Midlands have historically been dominated by fire-breathing individuals, who gained said abilities from consuming salamanders over generations.

Danger

The immediate danger of Fire gaining a sudden and massive foothold is rather obvious. More insidious is the long-term sympathetic feedback loops that unstable fire connections can create. Take the Molten Desert- once expansive and fertile plains, now sinking into the ground and getting hotter by the year. An area truly claimed by Fire is heralded by the arrival of fire spirits- ordinarily fleeting existences in mundane conditions, given time to grow intelligent and dangerous. The Molten Desert in particular is lucky in that glasswyrms consume these nascent spirits before they can truly gain a foothold, but they will eventually be outpaced as the region grows hotter.