I don't think adding instruments is a good idea for Guitar Hero. The last thing we need is 2 pretty identical products on the market: Guitar Hero's focus on the guitar with song choices, game structure and note charts is the thing that makes it different from Rock Band: it's aimed at a much more competitive audience, and to use a stereotype, the ScoreHero audience.
However, by introducing other instruments, you're having to be more careful about song selection, there's also a worry about notechart quality, will it 'dumb down' a bit like Rock Band for more accessibility?
Or, if you keep the Guitar the main focus and chose songs around that, you end up with a game which has singing and drums tacked on for the sake of a bullet point on a spec list, with them feeling really out of place.
I do like the Create a Create a Mode direction they're taking though, especially create a song, if done well that could be a really really good feature.
But a last point: every recent corporate move from Activision shows a very selfish, self-centered company, with Guitar Hero peripherals in the past being a part of it. I'm not going to buy a second drum kit or more GH guitars, or a Guitar Hero Microphone (because you know Activision are bastards enough to actually make you have to use their microphone).