Creating a Large Sound Stage

lack of strong direct reflections, diffusion between them, excellent bass. 

Diffusion to the sides is also usually a good idea. It's preferred to pure absorbers in some cases, especially for imaging. 

lack of strong direct reflections, diffusion between the speakers, excellent bass. 

Diffusion to the sides is also usually a good idea. It’s preferred to pure absorbers in some cases, especially for imaging.  Absorption on the floor and ceiling are often missed but very important. 

lastly, proper toe-in.  this is often less than people use.  That is, audiophiles will tape a cat LED pointer to their speakers to point them straight at their heads when many speakers are designed to play straight into the room. 

Speaker placement and listener placement are the most important elements in good soundstaging.  Whether that gets you to sound appearing beyond the speakers is then a matter of whether the recording has such information that your system is capable of delivering.  
Some recordings deliberately manipulate phasing and timing of the signal to generate such imaging, but, how effective this is depends on how well you have set up your speakers and whether or not your speaker design messes up such timing cues.  An example of a recording utilizing such manipulation heavily is Roger Water’s “Amused to Death.”  
Placing speakers as far as possible from reflecting surfaces and not having such surfaces between the speaker and listening position helps (i.e., no coffee table in front of your seat).  Adjusting toe-in is also important—the more toe-in the stronger and better defined will be the center image but the sense of image width will suffer; you need to find the right compromise.

Creating a Large Sound Stage

Constant Directivity Horns!

Mike

Everything matters, but some speakers just image and throw off a big 3D soundstage better than others so it greatly starts with that, and placing speakers too close to walls squashes subtle things like reverb trails that contribute greatly to soundstage size and sense of space.  So everything matters, but starting with speakers that have these qualities and positioning them properly is a huge part of it, and of course room/treatments also come into play. Just my $0.02 FWIW.