what is the room on the backside of it?

Powder room

Here is another angle. Powder room on the left, then weird triangle, then kitchen on the right.

How about a cool open wine closet?

If you added a door would it be large enough to store cleaning supplies? Is that an outlet in there? Good spot to hang up the cordless vac.

I'd either turn it back into a broom closet or cover it over.

I would put a door on it and use the space for bathroom supplies since it's right next to the powder room or maybe a place to put extra kitchen things that you can't find a home for like large pots/dishes/gadgets. Once your renos are done and you have a feel for how the space is really used, you will have a better idea what you'd like to put in there. Hope that helps!

Give the space to the powder room.