Its your decision, but many games chose a boss to include it in teaser-gameplay (e.g. see Dark Souls 3).
I don't want to make simple game, but it won't be a game with 10+ hours of gameplay with tons of NPCs (I plan to have something like 6-8 hours).
Just wondering about that. While 6-8 sounds enough for a 1-man-game, i would say that the time to create gameplay content (if the required tools are developed), is small in proportion to the overall dev-time. Your data-driven-philosophy should go hand in hand with that anyway.
You seem to be at a point in dev where you might want to plan the next years instead of a few month so how are your future plans? I could imagine some kind of episode-format?