Many were hoping that the Assassin's Creed movie would be the one to move video games into the realm of comic books and novels as serious and viable source material. That hope was dashed once most actually saw the movie. The movie was critically trashed and was also a commercial flop. The original plan had been to turn the movie into a franchise, but now those plans look unlikely. Star Michael Fassbender has an idea about what the movie did wrong. He says the film took itself far too seriously and took too long to get moving. According to Fassbender...
I would make it more entertaining, that's really the main note. The feeling of the film, I think it took itself too seriously and I would get to the action a lot quicker. I think there's three beginnings of the film, which is a mistake.
Needing to make the film "more entertaining" certainly covers a lot of ground, but the film's slow start is certainly part of it. The movie opens with a flashback sequence showing Michael Fassbender's character a young boy, then jumps forward to his execution and kidnapping by the Abstergo organization, and then finally jumps into the interesting part, the technology that allows his character to live through memories of an ancestor during the Spanish Inquisition. The movie certainly takes its time getting there, and could have likely opened much later in the story without sacrificing much.
By comparison, the first game in the Assassin's Creed series drops you right into an adventure by an assassin during the Crusades. The fact that the events you play through are actually a "genetic memory" isn't even revealed until the player is well established with the game. The games are about playing through the character's memories through history, the modern-day elements are a subplot. The movie swapped these ideas, making the plot about the modern day, and making the Spanish Inquisition a secondary concern.
As far as the film taking itself too seriously, which Michael Fassbender tells Movies'n'co was the case, it's difficult to say if that was the issue. The games certainly do take their plots seriously, and the franchise has remained popular doing so, perhaps there's something about the transition to film that makes the plot of Assassin's Creed a little more difficult to handle straight, though certainly if the movie had not taken itself seriously it would have been criticized by fans of the games for changing the source material too much.
Whether Assassin's Creed will have a chance to rectify the issues of the movie in a sequel remains to be seen. Michael Fassbender says it's up to the game's developer, Ubisoft, to make that call. Fans of the franchise would almost certainly love to see a sequel if it meant learning from the mistakes of the first one, though it may already be too late.