If you're designing/making it yourself, put your shelf on 4 sturdy casters that can just roll out, to give you the most use out of the space. https://www.pinterest.ca/pin/484066659920200756/

Yeah, you could do a triangular set of shelves on wheels, which would give you access to every inch of the space. Or you could switch the door to the side wall, which would give you three walls to access...

And put some serious storage on the back of the door for the stuff you use all the time (whether it's pantry stuff or cleaning stuff, or perhaps both, depending on what will fit).

For mops and brooms, to keep them from flopping on the door when you open it, use the new Command broom holders and put one at the bottom too--I did that w/ my back-of-the-door Swiffer, and it was BRILLIANT (if I do say so myself).

There are other options for pantry type things, but I'd vote to stay away from things like shoe pockets or even those Elfa baskets, and go for something deeper and sturdier. More like this: (In fact, look at that--is has a pull-out unit at the bottom--I bet that's an option for you!)

That shape of space is pretty unsuitable for many uses. I have one very much like it in my house.

You did not specify what area of your home this space is adjacent to and whether the idea is to keep things there that are to be kept as easy access, or just to store for occasional use.

If you store things for occasional use you could use stackable plastic bins.

It makes a pretty good space to stack those extra dining room chairs that you only use for holidays.

Stacked bins, extra furniture or shelf on wheels as was suggested----or, just keep it empty.

I really don't think that putting individual shelves on the stair treads will serve you any purpose. It is much too awkward.

Here is the pic that didn't come through earlier.

This has big boxes on the back of the door. That pushes the front of the shelves in the closet back further, and it makes more storage on the door itself.

But look in the closet at that unit on the bottom--it has casters; I bet this is an under-stairs cabinet, and that the unit on the bottom can pull ALL the way out so you can access the stuff in the back.

Now--you need to be able to open that door ALL the way up so the boxes on the door will be clear of the pathway for that bottom compartment.

If I were to do this, I'd have that rear portion of the bottom "cart" be accessible from the top or side.

jaye, do you have stuff that needs a place to be kept, or is it just that you have an empty space that seems to be serving no real purpose.

I see that it is empty right now.

Are those wires attached to anything? Or, are they just being stored? Sometimes the electronics for cable and internet get mounted in spaces such as this.