In my review of Arcania, I've explained that the game was so fundamentally flawed, it had absolutely nothing in common with any of the previous Gothic games. To recap, it lacked the open world exploration and the sandbox of previous Gothic titles, a hallmark aspect of the series, was replaced with locked zones segregated with often illogical artificial barriers which relied on linear quests; it dropped non-linear story advancement and multiple solutions to quests in favour of a simplistic "on-rails" approach to the point it has little difference than a Call of Duty title; narrative quality found in previous games were also simplified for cookie cutter spoon-fed storytelling and with a character template full of "personal drivel" forced on the player character to the point of not even giving the player a chance to define his or her character through words or actions, negating the point of Role-Playing entirely. At least for a game carrying the Gothic title.
Now, it is no secret that expansions often come with greater freedoms in design than in their main games. In many ways, an expansion is as much a reward for developers as it is for the gamers. It's often a recognition by a publisher that the developer has done good so they might as well give some leeway with the stuff that wasn't desired in the main game. Good examples that come to mind are Gothic 2 (Night of the Raven), Neverwinter Nights 2 (Mask of the Betrayer, Storm of Zehir, Mysteries of the Westgate), Morrowind (Tribunal, Bloodmoon), Titan Quest (forgot the name of this one) or Grand Ages: Rome (Reign of August). All of these expansions did things that were rather radical to the base game. Some added complete new features, others focused heavily on the narrative and writing. So, when a fully fledged expansion (ie. not a mere "DLC") arrives for a game from a big publisher, you generally expect good things. Greater than the main game in execution, though understandably not in scope.
Quite sadly, Fall of Setarrif falls even on that front. It made an almost tragic game even more so. Locked zones segregated with artificial borders got even more partitioned and limited. Story became so linear, you often had to wonder why you were partaking in it at all. Absolutely no improvements to the base game mechanics. So, Fall of Setarrif, offering absolutely nothing new or different, is quite very literally a cheap attempt at a quick money grab, perhaps to try to alleviate for the poor reception and poorer sales of Arcania? Quite possibly.
It has been mentioned that Fall of Setarrif are only option one will ever have if one wants a closure on the Gothic storylines. I disagree. There is simply absolutely no reason at all to even take Arcania into account. It's best forgotten. If I were the owner of the IP, I would simply retcon both games and pick up where Gothic 3 left off.
However, if you, or your 6 year old son or cousin, actually enjoyed Arcania, I can see that you would most likely enjoy Fall of Setarrif as well.