What about “If I could turn back time” theme. Do a timeline of your life so far, all the things that have happened—serious stuff, like world events, science, politics, etc. and fun stuff, like celebrities, music, sports, fashion, television shows, fads. Drop personal stuff into the timeline, like school stuff, friends, family vacations, pets,
Starting on your 15th birthday, have your parents take a picture of you every day. You can use that as a “countdown to Sweet 16” graphic, and use some of the images for the invitation. Decorate the space by years, with pictures to represent the events, fads, etc. and personal stuff about you.
You could have your guests come as something or someone from each of the last 15 years, like a costume party. Decorate the entrance with the pictures of you from this year, and use the pictures in flowers as a table graphic. Get a photographer or a photobooth so people can get pictures. Maybe have some pictures of you with a group blown up large and have face holes cut so people can be “dropped” into the picture.