So, where does this end? It reminds me of the Mach 3 razor, followed by the Shick Quattro, followed by Gillette's 5 Blade... how many instrument upgrades are we going to see do you figure, or is Rock Band and Guitar Hero going to ride these instrument packs for at least a few years?

I think they'll just keep adding something gradually every new release, something really small, but enough to make you want to buy the new stuff.

They may as well start packing some actual instruments with it.

Something like a keyboard or even larger drum sets to come next imo.

Well the drums are basically at their end point  with the release of the premium ion drum kit.  What more could you add-on to a real set of electronic drums.  But you will probably see most of the improvements made to the guitars and addition of new instruments.  

I think the next step (rock band 3, gh 5) is just going to be each one copying what the other did this time around

Hopefully no more... Im just buying the guitar hero game