This isn't excluded to those with songs in the game. Just say who you'd like to see in digital form.
I'd say Badmotorfinger-era Chris Cornell, from Soundgarden:
Not only is he insanely talented, but he also has a great frontman look with that long hair. Seeing him shout his way through something like Slaves and Bulldozers would be kickass beyond belief. The only big problem with this entry, other than the fact that I don't think Soundgarden makes an appearance in GH5, is that Chris Cornell has changed dramatically throughout the years. He's much more clean cut nowadays, and he might not want a videogame using his likeness that is no longer his likeness. Still, it'd be awesome.
As a bonus, I'll throw in another, this one also in his prime in the early 90's:
So, who do you all think should lend their likeness to GH5?