What an opening sequence should do :

An opening sequence is the first minutes into the movie, which is basically the 'make or break' of the movie, because if you excite and interest the audience from the beginning, in most cases, it means they won't leave mid-movie or that they will actually stay for the whole film. An opening sequence can be seen as :  (1) a beginning in a clear moment of action or interaction and (2) serving as a tease, revealing just enough information to ground the audience in the moment while maintaining enough mystery -- through the careful omission of certain information -- to keep them watching. I will have to consider and think about whether my opening sequence makes the narrative clear or flow or make it clear to the audience what the story line will mainly include and how you perceive your characters within this opening sequence because I do not want to bore them 2 minutes into the film. Other things I will have to pay attention and detail to would be :

Editing Techniques	This is things such as pace, rhythm, cuts & transitions. The opening credits are one of the first things the audience see once the lights turn off in a cinema, therefore, you have to edit it in a way that keeps them interested & grips them in. The audience can't bored when the film has just started. However, the editing must stay professional and only used when necessary, otherwise the opening sequence will look very messy, unplanned and all over the place.
Mise-en-scene	This is things such as the setting, lighting, costume and overall performance. When it comes to mise-en-scene in an opening sequence, it has to link in with genre so the audience can easily recognise and identify the type of film they are watching! It is also good at setting the mood, for example, if a lot of pink and shoes is included it gives hints that the film will be a chick flick!
Sound	Just like the previous point, sound is really effective at setting the mood and giving the audience an idea of the type of film they are watching! So if the genre of your film is 'Rock n roll' and the opening sequence includes jazz music...it wouldn't be very effective, and will simply confuse the audience. So sound/music needs to be chosen with great thought.
Titles & Graphics	This is overall to introduce the actors/actresses in the film and will add to the professionalism and realism of your movie.