For Master x Servent the idea was a high/typical fantasy world with certain wizards who can travel between planets using magic. A group of these wizards called the librarians work together to collect ancient artifacts scattered throughout the universe.

Among these artifacts are a series of seven tomes which no one can read but, according to legend, hold a complete account of the past, and future. When the librarians finally gather all seven tomes, one of the younger and more ambitious librarians attempts to steal the tomes. The traitor only succeeds in stealing the fourth tome, the tome of the near future, and quickly jumps to another world.

In this world he finds a young woman who wants to leave her life/world and agrees to take her with him on his travels if she agrees to be bound by magic to be his servent.

The only limitation on who the girl is is that she can't be able to world jump on her own. She may or may not know about "world jumping" and it may or may not have been discovered or brought to her planet.

Edit:sorry for any grammatical errors, I did this quick and dirty on my phone.