So, what is Fire-type's role, really?
The Legendaries and Their Role:
So far, there has been seven legendaries(plus Silvally and Arceus) that has been part Fire-type.
Ho-oh's name comes from houou, Japanese version of Fenghuang, a phoenix that represent the sun and the imperial family in Chinese, Japanese and Korean cultures that appears as the herald of a new era. Ho-oh represents rebirth, both metaphorical and real. It is also associated with autumn. Its title as the "Guardian of the Skies" likely is a pun on Sunny Day's Japanese name, Clear Sky.
Entei's design is based off Chinese guardian lions that protect Buddhist temples(like you know, Ho-oh's), and its name means Flame Emperor. Canonically, it is said to represent the flame that burned the Brass Tower, passion of magma and volcanic eruption.
Groudon's design based off Behemoth, who is said to be battle with Ziz, and Leviathan at the end of days to determine the fate of the world. Its Primal Reversion seems to be inspired by Canaan God Moloch. Both of its forms represent the land, specifically the plate tectonics that move freely on magma and all volcanic activities associated with that.Groudon also represent passion, according to Ruby (the item)'s flavor text.
Nobody really knows what this guy is supposed to be, really. Nonetheless, its name seems to come from heat transfer, and its overall design seems to be inspired by the molten core of earth.
Based off shisha, the Ryukyuan version of Chinese guardian lions, Volcanion seems to be a personification of geysers. Its ability to change the landmass seems to refer to tectonic uplift.
Based off Nike, Greek goddess of victory, Victini seems to represent the victory symbol. But its association with Fire is likely inspired by Moltres' initial role in the Victory Road, which inspired its role in the animé as the Olympic flame.
It represents Yang, truth and its tail is an electric turbine. It is a girly dude. Yawn.
More seriously though, its stylization as a torch is likely a reference to Prometheus' fire of truth.
The Trainers and Their Roles:
His most common Pokémon is Rapidash, but his signature is actually Magmortar, as of HGSS rematch(his original signature in RBYFRLG was Arcanine, his initial signature in GSCHGSS was Rapidash).
Holding the title of "One with a fiery passion that burns!", Flannery is a novice Gym Leader who took over her grandfather's Gym. Her Gym centers around Lavaridge Town's hot springs. Her Gym contains primarily Kindlers, a Cooltrainer(RSORAS)/multiple Cooltrainers(Emerald) and a Battle Girl, but in Emerald, there is a Hiker, and in ORAS, several Cooltrainers that appear in Emerald are replaced by Ninja Boys.
Her Signature is Torkoal.
One of the members of the Sinnoh Elite Four, Flint is the friend of the final Leader, Volkner. He calls the player to reignite Volkner's spirit. According to Volkner, the only reason he uses Fire-types is because they match his name.
His signature Pokémon was Infernape, but as of Platinum, his signature Pokémon is Magmortar.
His signature Pokémon is Simisear.
A member of Team Flare and the news anchor of the Holo Caster, Malva is the only Kalos E4 member that appears in the plot prominently, in the Looker sidequest. She is one of few non-villain characters that despise the player character in the franchise.
Her signature Pokémon is a female Pyroar, alluding to Lysandre's Pyroar. She also has a Torkoal, in allusion to AZ.
His signature Pokémon Alolan Marowak. The Totem Pokémon he is associated with is a Salazzle, and his Mega Evolution in Battle Tree is a Kangaskhan.
The bulk of the Fire-type trainers and Legendaries are associated with volcanoes, though volcanic Pokémon only cover a small portion of regular Fire-types. The bulk of legendaries are either death and rebirth, terraforming and passion or success and enlightenment. The bulk of the trainers associated with this type are passionate people of one kind or another, though vast majority of them have been unsuccessful in one way or another(Blaine failed the Mewtwo project, Chili and Flannery are novice trainers, Flint is too cool to give much a sh** to anything, Kiawe loses his Job in Sun and Malva is a bitter b... badass).
It is hard to say when the type will move on from its association with volcanos and burned places and instead focus on other elements of fire like cooking, lighting, spring celebration and what's more.