Shrek is one of those films that has had many sequels, and Dreamworks animated franchise will not die amongst its own fans. In the past decade, there was a Shrek movie every three years, starting with the original in 2001 and then the "last" with the Shrek Ever After in 2010. It doesn't look like that film is going to be the last one, as Shrek 5 is underway. Eddie Murphy has talked about it, and it is only a matter of time when the release date is formally announced.
A recent quote from Eddie Murphy on Cinemablend stated that "they took a nice long break...the next Shrek...I think Jeffrey [Katzenberg, the producer of the films] told me 2019? 2019 or 2020, it will be ready. But they've got a good script, they wrote a nice good script."
So they have a script for Shrek 5? I suppose that we shouldn't be too surprised by that, as the films have been making a lot of money for Dreamworks Animation, who has been producing at least one hit computer-generated animated film per year since the days when Shrek 2 was released.
So, with a script written, it is time to see if the actors are on board. Chances are, they could get them doing the voicework now. Mike Myers, the voice of the big ogre himself, has actually done very few roles after the Shrek and Austin Powers series ended. There are rumors that a fourth Austin Powers film is being made, and his imdb page has him cast in a film called Terminal, but his character name is not known.
It certainly sounds like Myers is free for the role. Cameron Diaz is also not as busy as she used to be, so she could reprise her role as Princess Fiona. Antionio Banderas could also be back as Puss in Boots, who has already had his own spinoff movie, and that actually has a sequel in the works.
So, considering that the series has already wrapped itself up, what more can happen? Last time we saw Shrek and Fiona, they had just started a family, as well as Donkey and dragon with this family. It would be interesting to see Shrek's kids grown up, and perhaps some of want to go back to being the life of an ogre. Much of the story of Shrek is how he, in spite of being an ogre, has become civilized in this fairy tale world. It would be interesting to see his sons go in a different direction.
It is also possible that Prince Charming could make a comeback, as he was always a foe that didn't meet a terrible end, unlike Lord Farquad (who was eaten by a dragon in the first film). Since Shrek takes place in a world full of fairy tales, there is a lot that really could happen in it.
In other words, don't be surprised if there is a plan for a trilogy of new Shrek films on the way.