Memorie

[Someone]

What if you need to fail the quest? Some games have included such feature as well and made them more interesting than straight forward game plots.

I'm not sure actually. I haven't thought about that yet.

With some conditions you can make missions a bit tricky. Lets say you have to fight a group of monsters, which you just have to reduce to a certain amount of HP instead of killing.
The mission fails if you kill one of them and can not be repeated. Its a way to give the player a reason to think about their actions, instead of just simple kill everything missions.

I made a sidequest in my game where you can find a key for a locked door someone dropped. The player can decide if they give the key back for something in return or use the key for the locked door.
They benefit in both ways, just the result is different. There is also a chance to make quests where the player can unlock sidestories with such sidequests or hidden areas or such.