Planar flora and fauna (using the Fire Plane as an example)

The D&D rules are weird and geocentric. If you tried to run a campaign entirely on the elemental plane of fire, you would run into problems with various spells and class features that target specific types because by the rules as written every living thing on the elemental plane of fire should be typed as an elemental. Inspired by Planescape and The Traveler's Guide to the Elemental Plane of Fire, I would suggest a few changes using the elemental plane of fire as an example.

Drop the restriction on the elemental type. The elemental plane of fire is inhabited by beasts, dragons, giants, humanoids, plants, etc just like the material plane. They just happen to be able to survive the standard high-temperature conditions.

Drop blanket immunity to environmental conditions. While the standard conditions would kill a human being, the natives find it normal enough. Conditions on other parts of the plane, however, may be intolerably hot or cold by their standards.

To reflect a more nuanced view of flora and fauna on the planes a la Planescape, I would go so far as to add new tags solely for this purpose. These tags would be [folk] and [fauna]: [folk] is analogous to humanoid while [fauna] is analogous to beast. For example, in 2nd edition the hellhound was portrayed as having an ecology comparable to a mundane dog... like hellpup burps causing accidental forest fires.

Hellhound pup, native to Plane of Fire

You could also just populate the other planes with beasts, dragons, giants, humanoids, plants, etc and make a blanket statement that they survive just fine in the extreme environmental conditions. Conversely, this means they might be unable to survive outside those conditions. For example, a human native to the plane of fire might freeze to death in conditions comfortable for a human native to the material plane.

Alternatively, you could just make the other planes survivable rather than arbitrarily hostile. What is the point in having them if they aren't meant to be visited? If they're just sources of summoned monsters, then you should really just reduce them to energy channels rather than actual places. If they're places that can be visited, why is the material plane called the material plane? It's obviously not the only material plane.