There are so many ways to customize a tee-shirt, and plenty of wonderful ways to fit your wardrobe. One very easy way to do this is to make this be a wrap-around garment. If you have a graphic or design on the front or back that you are particularly attached to, you can still keep it, and make it into a wonderful wraparound tee-shirt at the same time. You will first determine whether you want to keep the front image, or the back image.
Measure down the front or back of the shirt- depending on where the graphic you are keeping intact is on the shirt, and cut straight down the from top to the bottom of the shirt. Now cut the neck edge all the way around the neckline.
You now have a kimono-style tee-shirt, with the graphic displayed on the back. You can leave your edges unfinished if you want, or you can follow this step and finish the edges: Either serge the edges, or do a zig-zag stitch at the edges. You can also do the traditional finishing: fold over, pinning as you go to keeping a Â½ inch seam allowance. Once you are done pinning your edges in, start to sew using a blind stitch, from the bottom of one edge to the bottom of the other. Trim the ends of the thread.
So your shirt will need to have some "ties" to keep it closed. If you want to use the same material as the t-shirt, I would suggest using the neck edge. Or you can cut the bottom edge of the shirt, and use that material. Then re-hem it or leave it w/ raw edges, depending on the look you want.
Now determine whether you will have a tie that starts at the front of each of the hemmed edges, halfway up the shirt, or if you would like to place your wraparound ties elsewhere- you can have them at the bottom, which creates another look, and would likely require another tie midway, and at the top. The choices can be fairly endless and are up to your own creativity. Once the seams are sewn, you can experiment by using a safety pin to move the ties around the garment and trying it on. You can also go for a kimono look and have part of your tie slip out through an opening in the side of one of the teeshirts sides. The possibilities are really endless! Another option is to leave the teeshirt hemmed on the sides, but open in the center cut, allowing a belt to close it. You can always do another one ;) Experiment and go with what flatters the wearer!