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It appears the third game will feature a keyboard. From the internet gossip it hasn't been confirmed if it will resemble a dumbed down piano style keyboard or an awesome 80's style 'keytar' (shown below).
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There's already loads of tracks available with piano/keyboard parts but no one knows if Harmonix will take the time to recode the tunes so they will be compatible with the new peripheral. That would be a huge task considering there's well over 1000 tracks available (on disc and to download)!
Whatever happens I can't wait to play Rock Band 3.