I see two basic starting points for modern RPGs:
  • Hacked tabletop wargames that start out like an MMO raid but where you can do anything including subvert the entire goal with things the designers couldn't have anticipated.
  • Interactive collaborative storytelling with rules for conflict resolution and that help inspire the stories.

The first one is literally what D&D started out as and its lineage is very clearly seen in most RPGs (4e just used a boardgame not a wargame basis). The second one is what the Storyteller system claimed to be and what a lot of modern post-Forge systems do well. But most games are a mix of the two approaches.