I can't give many inputs on plots I'm afraid, but what makes a good role-play is pretty subjective and an open question. Albeit, I can give some ideas from my experience GMing.
Preferable that the world that the players' characters are in to have weight. As in, the characters' actions and interactions have influence over the world. Large or subtle. It creates a sense of grounded immersion, and that's basically what role-playing is about. It goes hand in hand with the plot.
In my opinion, a good plot is a relaxed plot. You can create a detailed and intricate plot with chapters, but the players might deviate from certain plans. Not everything has to go according to a structure, so sometimes improvise.
If the player won't go into the dark witch cave and goes to the forest instead, surprise them with another event.
This is all just my perspective.
But really, what matters is if the role-play and story is fun for you and the players.
