My Tinfoil Hat alt-universe speculations:
I'd quibble; with people citing Anime/Magna; as I feel it's impossible to know what form they would take without D&D's early influence. And I don't see the average wargamer paying them much attention, because...
My personal opinion is; that a "D&D" type RPG as we know it would still have to come out of wargaming culture.
So a lot of how it would look would depend on the type of fantasy that the authors were into at the time.
My pure speculation: IF "D&D" were introduced Today...
LOTR, GoT, would retain a large influence. Conan would have some influence due to film, but I wonder how much other older sword and sorcery would have been influential, as by the 80's the 'High fantasy' Tolkienesque pastiche ruled the roost: Shannara, Wheel of time, and a few other novels and pastiches thrown in... There'd be some monster movie influence; Dracula, vampires, werewolves, and some other non-D&D derived films like Willow, Ladyhawke, beastmaster, etc, pitching in...
Potential classes:
Swordsman/Fighter
Strongman/Barbarian
Scout/Archer
Monster Hunter/Huntsman
Priest/Holy man - Healer/Alchemist...
Rogue/Thief
Wizard/Sorcerer - IMO very much a Gandalf/Harry Potter mashup - Lots of staves, wands, potions, and rituals.
Some kind of 'face' class - Smuggler/Fixer?
I think the monsters might be drawn far more from Folklore than current D&D.
I'm also not sure we would have had the huge zero to hero power curve that D&D currently has.
Religion in the game would really depend on the predilections of the authors. But I can easily see more of a leaning into Alt-Monotheistic mythology's.