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Currently the single player game is like a predictable one way route. The goal is to go to the end world and kill dragon. For this we can find start-end step-by-step guides on youtube.When a player ends the game, he won't restart because it's boring because everything has to be done in the same way as before. That's why minecraft is mostly good only for multiplayer playing where everything is less predictable and less routinized.
For better single player game player should work something additional and more randomized before we could be able to find an end portal. My conclusion: create a lot of scenarios, one of them would be selected automatically and randomly at the start or survival or hardcore game mode.Example of scenario: a friendly mob comes to me asking me for help to kill a hostile animal. When i finish this, after some days another friendly mob comes and ask for help etc. Every next job is more difficult, at the end you receive location of only one end portal in whole world. Location of end portal is rendomly created when all jobs are completed.
You can add some things from the sims: find a woman, have kids, enable aging, give commands to woman and kids (e.g. they could seed vegetables), enable talking, adult kid can be player's ai co-warrior. When player's character dies, player can take his kid's character and so on.
Sounds boring, and pointless: Minecraft is already unrestricted and all this does is add unnecessary constraints that many people won't want. Spoiler alert? If you want to pretend to have quests, nobody's stopping you. But this would actually make the game less fun for people that don't have your very specific tastes. Minecraft isn't linear at all; it is literally a sandbox game, and the fact that there's no one set endgoal or way to reach it is a good part of the appeal.
The game has so much more potential than what you seem to be using it for. Don't restrict yourself to playing in one very limited way and make your lack of scope the game's problem.
When I think "scenarios" I think of starting the game in a challenging spot, for example all desert or nether spawnpoints. Currently these are impossible (well almost with deserts) because there is no way to get wooden planks, the first toolset. Adding ways to craft wood out of sticks or having iron tools in nether loot chests would help make these scenarios playable. I think this would be a far better solution to people getting bored in singleplayer.
That being said I don't think many people are bored after one survival world. There is so much to do in a sandbox game. The idea of an animal coming up and talking to the player sounds strange, if you want quests have them be villagers. But forcing the player to complete quests is even worse. And the Sims and Minecraft are completely different games, adding it to Minecraft is a different thread in itself.