Since Thor and The Avengers, Tom Hiddleston has gained in popularity by leaps and bounds, and Loki is probably his most popular (and well known) character to date. It is probably pretty well known by this point that he will be reprising his role as Loki in Thor 2: The Dark World and that he will not be in the next Avengers film. One of the things that has been rolling around the internet since it was announced by Joss Whedon that he would not be adding Loki to the cast of Avengers 2 was that Loki should get his own spin-off film. While it is very true that this "grass roots" effort to gather enough momentum to get a Loki-centric film was created and has been spread around by fans of Hiddleston, it does not seem likely that such a thing will actually happen.
The thing of it is, what makes Loki work so well is not only the acting ability and charm of the actor who is playing the part (or the very obvious chemistry that he has with Chris Hemsworth, who plays Thor) is the fact that he is not at the center of the film. As fantastic of an actor as Hiddleston may be, and as much fun as it may be to see Loki mess about with the minds of the Avengers (or his fellow Asgardians, and Thor in particular on both of those counts), the fact is that the way that the character has been written, he would not work as the central character in a film that followed him almost exclusively.
Part of what works so well for him is that we only see bits and pieces of him . . . we get just enough information about him to make him a compelling and fun bad guy for at least a few films (or what may turn out to be two films, and a complicated anti-hero in a third). Knowing that he has always felt like he has been living in the shadow of his brother, and has never been able to measure up to anything that Thor did (even if it was all only in his own mind); knowing that he tried to get the attention and the love that he thought that he was without, even though it was his the entire time; knowing that he has a giant chip on his shoulder from it all . . . all of that is what makes him attractive as a villain. But all of those things only really work when he is not the focus of the film, when he is the one who is working behind the scenes and making the heroes have to scurry around and clean up all of the mischief that he is so very good at creating.
So, even though Loki is a fantastically fun character and villain, maybe he should be left in the role that has already been given in the Marvel film franchise. It might not be enough for some fans, but sometimes "less is more."