Are you asking for ideas for storage or ideas to use this as a reading nook? Harry Potter didn't like his space either.

Unless there is some natural light, this area will revert to a storage space. With a solid door, this will always feel like a closet. I would remove the door, and maybe put a semi sheer curtain over it. something like this

Wiring a light fixture or wall plug into the space is a good idea, no matter how you will use it in future.

A photo of your actual closet would help.

https://www.hgtv.com/design/design-blog/design/nooks-niches-heres-how-to-optimize-those-quirky-spaces

No, not a reading nook. I'd just like blanket storage ideas. It's long skinny L. My basement is torn out due to flooding, but I'll try to get a picture.

Definitely do not want to remove the door.

Restoration crew here pulling out carpet due to a flood this past weekend. Anyway, this is the closet. The stairs to the right are taped off. There's a built in desk area on the other side (out of shot) to the left, so no side access to the closet. It's such an awkward space.

Dark cave inside.

You could find dressers (with drawers) that fit in that space at the back and put them on casters. Just roll them in and out. Or even just pieces of wood with casters on the bottom, and pile them with storage boxes. Then just roll the storage out when you need it. Add handles or a rope to just pull it out.

More easily reached storage can be in front.

Mentioned you were interested in blanket storage. Just needs organization.

Sorry about the flood but honestly we need to see your space to know what to do with it . The pic you show with the post has no need or place for storage I find it hard to suggest anything with no pic and no idea of what you need for storage.I need to see the stairs and how this space relates to them. Do you have a dog? How old are your kids ? The more info the better.

@Patricia Colwell Consulting As stated in the original post, my photos of my space are in the comments.

Also, as stated in the post, my kids are getting older, so yes, I have kids. They do not need the space to play in. They are 20 and 18 now, 6'4" and 6'7" tall. I have a dog but she does not need this space either. We just want to use it to store blankets but the blankets get heaped up and it's awful to crawl back there.

Thanks everyone. The blankets we have are big queen size comforters. No small lap blankets or anything like. We're primarily trying to store bedding for when guests come, so there are big comforters and pillows. Nothing small and tidy to fit into bins. As such, I don't think any of those storage options really work for us. I think what I really need is a regular closet with deep shelves.

I'm considering having shelves built that just wall off the back 'cave' space as it's totally unusable for us. Something that could be pulled out easily if the next homeowners wanted access back there.

I suppose something that rolls might work, but we had a really thick, lovely memory foam pad & carpet down here and I plan on replacing it with the same, so wheeling anything in and out of the closet might be a pain.

I would put sheet vinyl in the closet, specifically so you can roll bins of blankets in and out of the back area. Carpet has no advantages in that space. You can probably get 2 or 3 thick queen comforters in that back area in a wheeled bin. Easy to get out

We had a similar space in our split level house - a U shape, if you include the area under the lower stairs.

Under the lower stairs, I stored things that didn't need ready access - the big box with the artificial Christmas tree, and smaller empty boxes.

The space under the landing was 8' x 8'. I had 2 short bookcases. One was used mostly for house paint, stain, etc (which can't go in the garage or furnace room); the other was larger containers of cleaning supplies and miscellaneous stuff. The rest of that space was used for bulk things: golf clubs, power washer, and window screens in the winter; 2 large tubs (stacked) full of off season clothes and Christmas decorations.

Your space isn't as big - in our house, it was under the foyer - but still sufficient for some big tubs of blankets.

Inside the door, I had a bookcase on one side (shorter than the angled ceiling) that had textbooks and DVDs, a card table and folding chairs, and the vacuum cleaner.

I have exactly this closet under my stairs. I bought some wire shelves from container store and put 2 sets together (4 stackable and adjustable in each set) on the right wall when you walk in. I have clear plastic bins for cleaning supplies, vacuum accessories, etc. On the back wall I put another one of these shelves to store paint. Also on back wall I have a very large container I store season wreaths in. When you turn the corner (the part under the stair landing, I store blue air filters and a big printer and copy machine we aren't using now. I also keep my Meile canister vac, steam mop, and stick vac in the part that's walk-in. My husband installed lighting that lights up the whole thing underneath when we had the landing of the stairs open. So I can see everything in there.