My thoughts go back to, what were the general conditions that gave rise to D&D?
And yes, there's a huge shift if you assume that RPGs don't arrive until 1977 or later.
As it stands, D&D has a pretty clear formative basis:
- Centered in the historical context of the early 1970s (important because Star Wars didn't come out until 3 years after D&D was first published).
- Based on wargaming, with a small but fanatical preexistent base of fans.
- Despite EGG's protests to the contrary, it clearly took great inspiration from / borrowed from a hugely popular, existing work of fiction (Lord of the Rings) as a base on which to flesh out its inner zeitgeist and assumed milieu.
Now considering those things, I would say that if EGG and Dave Arneson had never been born, there are two sources of fiction that were popular enough at the time to fill in the gap.
First, it might be recency bias, but I think Dune could have easily been brought in to wargaming space. It's a natural fit to model its House Atreides and House Harkannon tanks and gun ships. At which point, I think an RPG would have been created and quickly evolved into something closely resembling a hybrid of Dune and what Warhammer ended up being 10 years after D&D was introduced in 1974.
Second, the most popular movies at the time were the sweeping crime dramas like the Godfather, Chinatown, and The French Connection, as well as heist movies like The Sting, and the enduring popularity of Hitchcock thrillers.
So I think the second obvious answer would be if a seed got planted in the wargaming community to build games based on 1920s/1930s mobster street wars, and then have RPGs be formed as a base from that.
With that in mind, it pretty quickly goes down the road of an RPG that's investigation-based, maybe a proto Call of Cthulhu without the Cthulhu mythos at first, until Call of Cthulhu proper actually shows up.
But yeah, if you assume RPGs as we know them don't come into existence until after 1977, the only right answer is Star Wars.