Harmonix was always the creative force behind Guitar Hero. They developed the game. Activision was the publisher in association with RedOctane. Harmonix has been making rhythm games for almost ten years (see FreQuency). Activision may have been taking a risk in publishing the first game in the series, but don't mistake that for innovation.
Technically, Activision is still just the publisher, with Neversoft heading development, but whatever.
I'll give you that, but that is often the way with sequels. I'd say that Guitar Hero 2 refined the formula, improving the hammer-on/pull-off mechanic among other things.
I wouldn't say Activision is set to surpass Harmonix's ideas. Other than a few added details, World Tour looks like little more than a blatant attempt to turn Guitar Hero into Rock Band with a few extra bells and whistles. The controllers look nice and the music editor function piques my interest (we'll see how well it turns out in practice), but I wouldn't call either one of those especially "innovative."
One of the things I'm curious about is how World Tour will handle DLC. Harmonix has released at least three new tracks every week since launch day, sometimes a full album's worth (just today, Harmonix released a Pixies album and three Weezer songs). Their DLC practices have turned Rock Band into more of a platform than a game. You don't own Rock Band, so I don't fault you for not knowing all of the game's strengths, but don't assume that World Tour will surpass everything Harmonix has accomplished with Rock Band.
I expect World Tour probably will sell more copies than Rock Band based on the strength of the brand alone. Rock Band just doesn't have that kind of popularity yet. I still don't see any "innovation" on Activision's part, but I'm interested to see how well their product turns out.