Branches that grow by themselves

Should you write a fantasy trilogy? This article brings up really good points to ponder, like the pros and cons of making that decision before, in the middle of, and after writing a novel. The Shared World approach is my favorite - that way I can work with an already known universe, without having to stick to one specific plotline. An example from manga would be Saint Seiya The Lost Canvas Gaiden, stories about various characters from the LosCan manga, with bonus goddess kidnapping.