It is not appropriate. Fanfictions are an extension of the original work, but they should still be in line with the basic values of the original. The Naruto series promotes values such as perseverance, loyalty, and respect. A'slave fanfic' for Sakura or any character undermines these values. It can also be considered disrespectful to the original creator's intention and the fan community that enjoys the positive aspects of the series.
Definitely not. In the Naruto world, the characters are strong and independent. Sakura, for example, is a skilled ninja. Creating a'slave' fanfic goes against the essence of the characters and the overall positive theme of the series. It can also make many fans uncomfortable as it distorts the characters in a negative way.