When I watched the Lego Movie, I was taken into this amazing Lego world, but towards the end when it is revealed that it was actually just someone's Lego set, I was transported to a different world. The Lego Movie is great, but now that I know that it was all just in the imagination, it just will feel wrong to enter into the magical Lego world again.
They would be forced to have a different story. No one can make a movie about dumb block sets (the cliffhanger) and solve the problem. It was meant to be funny, not serious. A sequel with a different story is totally not needed. And a movie like this is insanely hard to create. Hey, it's my favorite movie, narrowly beating down Aladdin!