In an old interview from 1997 i think (shortly after the release of City), Devin was talking about a "Hell" project (can't remember the exact name). He was talking about some really extreme music, a lot more extreme than "City" was at that time. Did you ever heard about it? Did he actually made some tracks or was it just a project he finally decided to give up?
There's also the power metal project, the puppet show, the temperamental...book? thingy, all kinds of things. Devin seems to be one of those artists that discusses his ideas openly! Unfortunately it's great to speculate, but he's a busy man and thus a lot of these ideas will probably never come to fruition.
I don't mind, though. I've come to expect this from a few bands/artists. You learn to just take them as they come and shut up and wait! =) He also once said there would never be another SYL album!!