Ever heard of the Chiodos? They'd be a good one. Maybe some proggy type stuff... I'm always a fan of more A7X, and we know it'll always be a good challenge, if they pick a good song. Really, if you want harder stuff, guitar virtuosos that seem to think they don't really need more than a vague backing band (and they'd typically be right) and Power Metal seem to be the way to go. More Dave Mustaine stuff, maybe. You see where this is going, right? Drums, you really have to dig into what's harder for a GH Drummer. Gene Hoglan is a beast, whereas Mike Portnoy becomes a total mindfuck. Did I mention A7X? Vocals will always be dominated by Queen, but maybe Would? by Alice in Chains? And Bass will just continue to benefit from songs that either don't directly follow the guitar or follow the guitar so well it actually becomes harder than the already-seeringly-painful-to-play Guitar Track. Maybe a rare integration of that lovely Turntable peripheral they made such a big deal about and then threw away at the first sign of shit going down in some Linkin Park songs or Public Enemy or Run DMC... Oh, who am I kidding, Acti isn't gonna even try to bring that thing back. XD
Anyways, those are a few ideas. Say what you will.