Kids have an abundance of imagination. My daughter builds the theme sets but then all the characters from all around the house play with it. That includes Lego minifigs, Lego friends, Lego princess, and a tonne if playmobile characters. The games she thinks up with them are much more involved and elaborate than the sets limitations.
This is her current setup. It is a mash off medical castle, Lego friends house, elven time travel castle with a. Ice cream truck for good measure. Also check out sleeping beauty’s bed in the turret of the castle. For good measure there is a rainforest animal clinic in the backyard of Emma’s house.
I think to say the theme sets stifle imagination is a bit harsh. It lets them bring the world of cartoons and superheros into their imagination in a way the movie makers could never have thought of.