I'd like to request Richie Sambora. He has a new solo album coming out next month (on an independent label) and has always been supportive of Rock Band and music video games in general, so I think he could be persuaded.
His back catalogue is fantastic too - think Bon Jovi but with a more blues-influenced sound. His first album, Stranger In This Town, is my favourite album of all time. I would die of happiness if we could get solo Sambora in RB.