It is the rare mom who would limit imaginative play at home. However, there might be some play resources to add to your home. Please add your favorites in the comments if I missed them!
Pillows - This is my "go to" gift for the boys. I recommend getting all sizes - body length, couch (that they can play with), throw, regular size, etc. We have had pillow pits, forts, dog houses, etc appear in our living room.
Sheets and Blankets - These help extend the pillow play and can be used to make a variety of dress up outfits. Different colors can encourage play as well - blue for water, brown for desert, etc. Yes, please get the play sheets from a thrift store. In a pinch they can be used like a parachute and keep balloons in the air.
Beanbags and balls - These can become a variety of items in play from bombs to food to currency.
Play Silks - These are the fancy, smaller version of sheets. I actually don't have them but they come highly recommended - they can also help with active play with music. Typically they are smaller sizes so easier to use for clothing.
Cardboard boxes - We have all witnessed kids play with the box more than the toy. Try to keep a variety of sizes on hand (larger encourages more creative play) on hand.
Crazy Forts - This helps those sheets and other items stay in place better. I have seen friends create boats, forts, stages, and a variety of other things out of these things. Tons of play here.
Dress up clothes - I would recommend things that are more generic - not a specific character. My boys actually LOVE their body suit - it's red and they streak around our house in them pretending to be all types of characters.
Tents and tunnels - We currently have a small tent in our house. Inside or outside, it adds fun and creative possibilities.
Playstands and child sized tables - These are large wooden structures that kids can use to make a variety of things: play house, grocery store, really anything. They are a little pricey but I imagine you could make sometime similar for much less. Here is Ana White's version. You might be able to make something similar out of PVC pipe as well.
As children get older I would encourage some skill based, open ended play. By providing them with some basic instruction and time to tinker, develop and play they can grow in confidence and skill.
Tinker and Loose Parts - Providing your children with a variety of tools that they can explore.
Sewing supplies - needle, thread, thimble, crochet hook, yarn
wood play - hammer, nails, screws, screwdriver
art activities - paint, paper, colored pencils, brushes, crayons, etc.
pvc pipe also has lots of possiblities
Tinkerlab has a long list of supplies you can use.
I would also count wood blocks, legos, snap circuits, playmobile, Little Bits and similar items as toys that encourage creative storytelling, play and building.
I am sure there are many more resources. Check out your local thrift store and get some clothes (fancy and plain), old kitchen supplies, sheets (I am not sure I would get pillows - depends on the location), balls, craft and tinker supplies and find a way to organize it and make it accessible.
Remember that if kids are used to playing with something electronic or based on a character it might take a bit of time and practice to enjoy the more free form play.
I will admit that this is easier to do if there are other kids around to encourage the story making and extend the tinkering ideas. You might want to check out the previous post about creating space for outdoor play and consider how some of these ideas might work for imaginative play as well.
Food for Thought:
What is your child's favorite toy or game? Why does he/she like it so much?
How many of the toys in your house are open ended? How many are character driven and single use?