In brief, I saw elementals as synthetic entities created by sorcerers as needed, or occasionally generated by accident when powerful magic interacts with ambient elemental phenomena. (Say, a sorcerer blows a control/activation roll on a weather-control spell and generates an air elemental, or blows a spell of any sort out on the ocean and generates a water elemental. Makes Side Effects more interesting.) There are no "natural" elementals, but some of these spirits were created so long ago their origins are forgotten. There are also Outer Planes with strong "elemental" character, and the entities that dwell on them could be called "elementals." But those entities have no connection to Earth. If someone wanted to postulate elemental planes for the CU, I'd make them very abstract and arcane, like Yggdrasil or the Blood Tide -- accessible through the Lower Astral but difficult to reach and even more difficult to comprehend. Visiting the Plane of Fire would be like visiting, I don't know, the Higgs Field. Not "It looks like Earth but everything's made of fire." You want that, look in the Outer Planes. Dean Shomshak
I happened to be reading through a non-Champions Hero Games book, The Valdorian Age by Allen Thomas, and came across something quite interesting and potentially relevant to this issue. For those who are unaware, the Champions Universe of the present-day era is officially part of a much longer timeline, the "Hero Universe," spanning many millennia and ages of civilization, from the prehistoric past to the far future. The Atlanteans, Lemurians, VIPER's patron Nama, the Archlich Takofanes and the Crowns of Krim, all have their origins in these past ages. The Valdorian Age is Hero's "sword and sorcery" setting, evocative of the fantasy fiction of Robert E. Howard, Fritz Leiber, or Michael Moorcock.
Part of the VA sourcebook describes the sorcery of summoning and controlling elementals, and presents an interesting conception of the Elemental Realms. Note that this is only as Earthly sorcerors of this era understood cosmology, and it's perceived through the lens of creation-myths of the day. That day was also more than 40,000 years in the past, so even on other planes the conditions might have changed:
"The god Pythos created the world, but like any craftsman, he only shaped it -- the materials for his work came from elsewhere. The source of these materials were the Elemental Realms, the first source of Fire, Air, Water, and Earth, where the Primal Lords rule: the Scorched Lands (fire), ruled by the Kiss of Flame; the Howling Void (air), ruled by the Voice of Wind; the Infinite Depths (water), ruled by the Roar of Waves; and the Vaulted Warrens (earth), ruled by the Groan of Stone. The priests say Pythos struck bargains with the Primal Lords to obtain the materials; others say Pythos tricked and swindled the Primal Lords out of the materials.
... Unlike demons, elementals are not separate entities (at least according to most sorcerors) -- an entire Elemental Realm is an entity unto itself, with its Primal Lord as its "mind." The elementals are simply parts of a "body" -- facets of a composite being. While individual elementals are intelligent, their mentality is wholly alien to mankind. They view flora and fauna like a man would view a cancerous growth, mines and tunnels as deep cuts and stabs into a man's flesh, water in a cup or bowl as a man in a cage, trapping the wind in a sail as a man in chains, smelting ore as maiming, alloying metals as rape. The concept of property baffles them, for in the Elemental Realms everything is part of the whole -- can a man's right hand own his left?"
- The Valdorian Age p. 131
There's another passage from the VA page I quoted above relating to elementals and how they would view our world, which suggests a unique and interesting potential motivation for a "supervillain" or villain team:
"... when a sorceror causes an elemental to manifest, it violently struggles to break free from the sorceror's constraints and rage across the world. It pleads with the sorceror to allow it into the world unchecked so it can free the substances trapped in the forms and shapes that comprise the world -- forms and shapes the elemental considers a corruption of elemental purity."
Imagine a powerful elemental, or a group of them, with this mindset, loose on Earth. They would level buildings, smash machinery, tear up roads and railways, shatter dams, cave-in mines; but also uproot forests, ravage farmers' fields, slaughter domestic and wild animals. To humans these beings would seem like total nihilists, or insane berserkers, but from their perspective they would be fighting to heal the wounds of violence, and purge the infections and parasites, that are destroying this world.
Trying to communicate with these creatures, and reach an understanding, could make for a fun roleplaying challenge.