A walk through pantry front offers a seamless look within the kitchen but maximum storage space in the pantry within. *Should consider efficiency in kitchen and whether or not a reach through pantry would be realistic for your needs.
Design Tour: A White Kitchen w/a Soft Look and a Whole Lot of Pretty
Build these handy undersink roll-out trays in a weekend. You can tackle this project with simple carpentry tools and some careful measuring. You can make all the trays in an afternoon using building products from your local home center or hardware store
Hidden doors -- This company makes cabinets, bookshelves and anything you can think of that fits your existing doorjambs thus hiding the door (and also making use of the space) Not sure I like the idea of a 'gun cabinet' door.