I hate the boring stuff like sell or package in collector's editions like art books and t shirts. There so many interesting things they could offer.
In no particular order:
1. Candles. They could come up with some wickedly evil Diablo-themed candles. Scented, unscented, maybe an evil-looking skull or sconce or candleholder to place them in. I'd love to light one when I play Diablo.
Something maybe like this except the skull has a soulstone in it
Or maybe an LED-flameless candle that can project images and scenes. Use it to project images from Diablo.
2. Leather-bound journals with thin cloth bookmarks of various colors. High quality. Diablo-themed. It doesn't overtly have to say "Diablo" on it, but make it look evil with skulls and maybe evil-looking charms attached to the ends of the bookmarks, or crucifixes on the end of the bookmarks. Something like this:
3. Necklaces. I have a soft spot for a good necklace. My favorite is my Doctor Who necklace of the TARDIS key.
I like it because no one knows what the hell it is unless they are an old school fan.
I'd like to see blizzard release something for Diablo. Maybe an evil skeleton key.
4. An Ice-Cube Maker - Where it makes Blizzard-themed ice cubes. For an example, here is a Star Ward themed ice cube maker that does Millenium Falcons