Well, isn’t this something. Coming straight off of Harmonix alumni @deliciousbees’ Twitter feed is a video from fellow Harmonix alumni Joe Kowalski (@codeloss). While he was working as a User Interface Artist at Harmonix, Joe helped put together a pitch video for Guitar Hero III in 2006 for Harmonix.
The problem here is that the Guitar Hero III Harmonix envisioned never happened, as the IP rights went with Red Octane to Activision before this game got off the ground, and Neversoft ultimately ended up making Guitar Hero III: Legends of Rock instead.
This game would have been a radical departure from Guitar Hero II, and a much different tone from the rest of Harmonix’s games.