Utopia is a cancelled chapter of the Guildwars Saga, it was supposed to be chapter 4, but got cancelled and Arenanet decided to make Eye Of The North instead which is an expansion and not a chapter.
The reason why Utopia was cancelled is still unknown and whether Utopia was going to be the link between Guildwars and Guildwars 2 is currently unknow as well. Although the actual story line is not know, many people derive a lot of information from the cancelled chapter by looking at all the released concept art. This Chapter was supposed to be released April 2007.
Concept art for Utopia made by Daniel Dociu
What we do know is that Utopia was going to introduce a new profession called the Chronomancer. The Chronomancer is in fact some kind of time mage, but nothing is know about his weapons/abilities. Utopia was also going to introduce some new races, like the Sidhe (which later became the Sylvari for Guildwars 2).
Concept art for Chronomancer
by Wei-Che (Jason) Juan