Pondered the "new" (OP) version of the premise, where RPGs themselves don't exist until now. I think there's a way to imagine it, actually, that allows for the seeds mentioned above to still be an influence, but account for the fact that they improbably wouldn't have borne fruit until 2021. You have to think about what the halfway point would be between those seed ideas and the RPG.
The catch, of course, is we're now looking to shape history to the outcome, rather than simply making a change (no D&D in 1974) and seeing where it would take us. But it can work.
So if CYOA or Advent or adventure games are the seed, you need to imagine how they'd become more sophisticated with time. I assume it'd become a matter of the games offering more and more choices, giving the players more flexibility to influence the outcome of events, but never quite making that leap to defining the setting or characters in objective game terms. In that path, I can see very complex storytelling games emerging, particularly in the computer arena, and becoming popular multiplayer options once the internet arrives. The players' inputs would be very flexible, with increasingly complex, simulationist software being expected to do the work of interpreting them into results. Then, once tabletop gaming starts getting bigger in the 21st century, there's a translation of that idea into a streamlined boardgame form, which would be our first proto-RPG.
I would expect RPGs in this path to be very freeform, only loosely defining your character, and probably focused more on player-invented backstory and what possessions or gifts a player has been granted during the course of the story.
The game system, meanwhile, would mainly be there to provide randomness and tell the players the results of their actions, without really shaping the way the players themselves behave. There might be meticulous lists of possible outcomes to player actions. The GM would probably be much more of a referee or facilitator than a storyteller themselves. Conflicts would be resolved very simply, perhaps taking inspiration from rock-paper-scissors as seen in some LARPs.
Campaigns might not be as much a thing, either, more the idea that you're all there to tell one story from beginning to end. Though "sequels" and such could certainly follow.
If miniatures wargaming is the seed, on the other hand, I would expect something on the opposite end of the spectrum, very very simulationist, since multiplayer computer wargames, with specific "general" or "warlord" characters, would be the likely midpoint. Honestly, it might not be that different from the D&D we got, if somewhat slicker in production, and with, again, less GM authority and more dependence on the rules to determine outcomes.