While browsing Instructables, I found a tutorial on how to build your own smartphone using Raspberry Pi and some other fairly cheap components on Adafruit. It looked fairly simple. The author had made his own operating system called Tyfone, and it could receive and send texts, call, and do most things a normal phone could do. It was a modification of the Raspbian operating system, and didn't look too hard to make. You can't download apps, you make your own, which I thought sounds pretty awesome. Plus, you can make both apps and OS using Python, which is my favorite programming language!
I actually already have a lot of the requirements for building a smartphone. Over the years (or months), I have bought small things for the Raspberry Pi like a camera, USB hub, adapters, things like that. The two big things I need now, is a portable power supply and a small touchscreen. Thankfully, you can get both of these things for about $40 each.