Things like a nice SPL meter and a guide for it would be awesome, especially for beginners trying to set up a really accurate system, which is obviously a big reason to choose av123 for the x-series anyways.
What I'd feel most strongly about, though, is the inclusion of speaker wire, especially high quality 12 gauge or something like that. Speaker wire is a very easy thing to muss up your system with, and for just a little help by including choice wires, I bet the novices and even the enthusiasts of HT would be happy with paying a bit more. I certainly would.
And stands would be wonderful, perhaps as a package discount of some sort. If stands are offered, however, as a big fan of wall mounting I think it would be important not to forget to offer simply the wall mounting panels as a bundle in addition to the option of stands. This would increase flexibility, as well as make it easier to manage ordering a complete set up all at once.