Creating a new closet

Our older house has limited storage, and we are considering every option to create more. There is a “cavity” in the second floor above the front door which I believe is intended to be used for decorative purposes.

How hard would it be to frame it in and open up other walls (either the closet behind it or the trophy case wall in front of it)? I understand there may be unforeseen challenges (load bearing studs, electrical, etc), but trying to get a feel for cost and effort.

Thanks in advance! https://imgur.com/a/cAoH6bm