I must admit I'm infatuated by the idea.  However I have a hundred questions concerning how this would be work as a theme park.  You can't have people carrying real swords in case they trip and stab themselves or others.  You can't have real fireballs or spells that make you fly.

Perhaps if it was set in the near future and high resolution holography has now become a thing?  Swords are just hilts, their blades are holograms?  Sure you can summon a stone wall that hosts would see as real and not be able to breach but a cheating player could walk right through it. That way a giant robot dragon could breathe holographic fire on you without killing you for real and it would be up to the guest to act out being injured?

Maybe there's an V.I. that controls the overall park and the holograms that might help out the guests when certain hosts start getting murderous? 

The other option, come to think of it, is go full on Star Trek mini-story.  A holodeck could control gravity and create forcefields.  The crew gets caught in their own D&D program, mortality fail safes get turned off, and something inside the program is waking up or the shop is falling into a star so they need to escape in a hurry.