I don't know what should be in a boxed set. I know what I'd prefer wasn't.
If it would work perfectly well in one book, don't break it up into several pamphlets, put them in a box, and sell it to me as if there's some value added.
If they're going to do a boxed set, this is probably the way to go - you need to play to the strengths of the format.Great point. Bouncing off of that, what worked great for the Essentials Kit is having a ton of board game like paraphernalia: handouts, cards, tokens, etc.
I'd go for something along that line: maybe some paper miniatures.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.