You got a great idea. Should patent it. Maybe not interchangeable. But auxiliary lens that you slide or clip into position on top of the existing handphone lens is a possibility. Particularly for telephoto or super wide angle attachments. You know, like the additional attachments they used to make for Yashica Electro 35. Most handphone lenses can handle close-up shots on their own. So maybe a super micro auxiliary lens that allow the handphone to take microscopic pictures - for example, 50x magnification.