I pondered your cupboard whilst on my walk.Without knowing what is contained in the lower section, you might investigate having the lower section removed, shelves installed in the full height, with a frame to fit a door and a full height door fitted. It could be an obvious cupboard, with fitted handle, or it could be fitted in a way as to be flush and be a secret cupboard with the handless latches used in kitchens.

If you stated what you planned to store in it? You'd get ideas. If that is a circuit breaker box on the right? you need easy access to that ..........

The same door as every other interior door in the house .

Where is it located? In view for all to see? You might be able to just put up shelves.

The space is too deep for useful shelves (the deeper the shelf, the more junk it collects and you won’t have access to the stuff furthest inside). I would remove the existing weak shelf first. Then decide what you would like to store in there before reconfiguring. If the covered recess is your circuit-breaker box, it must be readily accessible. You could put a shelf unit in the rear area (it would need a center support, too), or a steel shelf unit on wheels, and a heavy-duty rod if you need a place for winter coats. I would put double doors across it repeating the style of any other doors in the adjacent spaces.

if you have large items (suitcases, etc.) maybe they can go here where you have deep shelves.

That door leads to attic where the central aid unit is.

The space below the “floor” of the closet can’t go all the way to the ground bc of the ceiling of the stairway below. Id like to use it as a closet and storage for things. This is in an extra room where I have no plans for it as of now.

put in better, heavier shelving. we made a deep coat closet into extra storage like this. multiple shelves can store a lot, since it is deep.

see about putting a jamb around the opening w/a regular door, or try bi-fold doors

You didn’t specify what kind of closet, but it looks like a cubby inside a clothes closet, so with that I’d recommend installing a pull out pants holder on bottom and a closet rod above for hanging shirts. And I would definitely investigate Whats below in that space, you could add a shoe cubby below the pull out pant rack if it’s available. You can also install a pull out belt/scarf rack on the upper left side as an easy access to those accessories.

Well, didn’t see the access to the attic. Is this the on,y access? Looks small. Anyway, if you ca don’t hang a pants rack over the access just install a clothes rods and use for winter coats or hardly used items.

Sorry about the misspells, doing this fast ( at least that’s my excuse and I’m sticking to it🤣)

If you want to leave it opened you can wallpaper the space, then add shelving, etc...

We had something similar in a rental we own. During a whole house remodel last summer we had our contractor build some doors and trim to turn it into closet. I’m not sure what he did inside it though as I don’t have those pics.

I like the idea of turning it into a seasonal storage area for clothes etc. I have attached a photo with an better description of the space. If the lower area encroaches onto a ceiling but there is space there don’t waste It. Have a drawer installed so that you can fill it with season loose items. On the upper half. Where the shelf is right now that area has been reinforced add two hanging rods. One front and one in the back. For each season rotate the jackets etc to the forefront. This still allows you access on the rare time you need through that access panel.

Then above the previous shelf add a shelf that will fit baskets. These baskets can store baseball caps and mittens etc. Then frame it all out as a regular closet so it’s one large double door access to hide it but still keeps it’s functionality.

OFCOURSE it depends on what you want stored here ..but a few options would be more shelves in back and a door with a built in bookshelf... Which would take care of filling the gap between shelves and front space . It would be pretty to look at books and have a hidden closet behind . Or a simple door with a shoe rack on the inside built into the door . Or fill it with clear storage bins with whatever you don't use in season ..Parkers , boots , clothes ...or ...make it into a pantry ...with louvre doors ..
Have fun figuring out if you need things hidden or want to display a mini bar ..coffee station .....as long as you leave access to the electric box ...mirror it ..paint it black and put a sculpture with a spotlight ....so many uses ....!

No photo attached :-)