I just got the idea, but I don't have the stuff I need for the gimmick. I also don't know if it will work. Some info:
1). Need to make a new gimmick each time. (Easy, only 2 materials and some cards.)
2). All materials (there are like two) should be household, and if not, easy to obtain.
3). Not only can you change the signature, you can use it to create signed dupes.
4). Can move the signature from front of the card to the back, and if you want to, back to the front again.
5). Simple sleights a good magician will already know.
Hopefully the trick will work, because if it does, it will be revolutionary.