A prequel to Saw (unless it was Saw 2) wouldn't really be necessary as we already found out why John Kramer started jigsaw off and it's not exactly a detailed plan, couple of flashbacks in a few films kept it easy and didn't complicate that area of the story.
Could see this film possibly focusing on Lawrence Gordon now he's somewhat alone under the Jigsaw name, go back to when he left Adam and see him and John teaming up, but would be set predominantly in present day rather than having a Saw 1.5. I'd love to see Tobin Bell back in the long black cloak again, if only for a flashback!