I saw Chris Broderick fill in for Steve Smyth when I saw Nevermore, and he did an awesome job. And Jag Panzer are really good. I don't know if the rumors are true, but I dont see how adding him to the band could hurt the writing process.
I am also heard a rumor (which I think has been discredited) that Jeff Waters of Annihilator was going to fill in. I think thats false, but it would still kick a ton of ass.