Alright, i came up with a new quest. It was tested with OpenMW but i'm sure it works in vanilla Morrowind too.

Quest name: A Broken Family

In the Sadalvel Ancestral Tomb, near the city of Necrom, the player will meet a friendly vampire by the name of Verick Arthe. If the player decides to talk to him, he will tell that he once was a librarian in the temple of Necrom along with his brother Alain Arthe, but that he got turned into a vampire during one of his pilgrimages years ago and started hiding ever since. He then asks the player to find his brother in Necrom and to tell him that he's still alive.

There are several outcomes to this:

• The player finds Alain, Verick's brother, in the library of the temple of Necrom, and tells him that Verick is still alive. Since Alain is a faithful member of the temple who despises monsters and unholy creatures, he will ask the player to go back to the ancestral tomb and kill his brother. The player can refuse or accept this request.
• If the player accepts the request, two things can happen:
  • The player will come back to the tomb and kill Verick as he's told, then go back to Alain in the temple and get a reward.​​​​​​​
  • The player will come back to the tomb and decide to spare Verick instead, and then lie to Alain by saying that Verick was killed and still get the reward.
• If the player refuses, Alain will take this matter on his own hands and will decide to kill his brother Verick on his own. Thus, if we go back to the tomb, we'll see Alain and Verick fighting each other. There are several outcomes to this, too:​​​​​​​​​​​​​​
  • The player can refuse to help Verick or Alain, and let them fight each other until one eventually wins. If Verick wins, he'll start going mad and he'll attack the player. Instead, if Alain wins, he'll simply refuse to talk to the player. 
  • The player can help Alain by killing Verick. Once Verick is dead, Alain will thank the player and give him a reward in gold.
  • The player can help Verick by killing Alain. Once Alain is dead, Verick will thank the player and give him a special enchanted amulet as a reward. 
• Or, the player can kill Verick immediately after the quest is started, and he'll still get a reward if he meets Alain at the temple and tells him about what happened. 

If Alain is killed at the temple, Verick will disappear forever. If Verick is killed before the quest starts, Alain will disappear forever. The quest will end if the player kills both Verick and Alain while they're fighting each other.