Don't waste the space under your stairs.

http://96.30.19.132/~designone/wp-content/uploads/2013/01/workbench-under-stairs-storage.jpg

http://c0263062.cdn.cloudfiles.rackspacecloud.com/content/images/sized/under-stairs-storage-small-home_7a6282e5b2efb4661834933b02b60d5a_3x2_jpg_570x380_q85.jpg

http://stairbox.co/wp-content/uploads/2015/12/wooden-under-stair-storage-closet-design-under-the-stairs-storage-design-ideas-728x486.jpg

try a small woven box or basket with a lid? doesnt have to be a big piece of furniture.

You have an air return on the left & on the right you need space for the door swing.

put a rack inside that closet door.

I put a rectangular basket by my door. It has solved the issue like nothing else did. No extra steps to put shoes away. No bending to tuck shoes into a cubby. It's amazing what an obstacle those tiny extra actions are-- but they are.

My personal favorite is to hang a shoe rack inside the door; however, there is often a family member (or three!) who feel that it's just waaaaaay too much work to open the door to find or return shoes.

Another option may be to place a mid-height, narrow accent shelf to the left of the closet. You'd lose the lower half of your window, but you'd have a simple place for shoes to land where they aren't the first thing guests (or YOU!) see as the door opens. Place a unique, fun piece of pottery or vase on the top, and voila!

Note: Be sure to have a small dish for coins, keys, etc. As soon as a surface appears, it's bound to gather "stuff" as people are on the go.