Blu-ray of visuals from Tomato and music from UW would be ideal (hint, cough, hint). MMM 120Hrz 1080P, I luv U!
What I'd really like to see is the album continue on in downloadable content....have it fade out or stop right as something cool is happening, then release 30-45 minute mixed track EPs on a timed schedule (similar to RiverRun project) that picks up the album where it left off and continues it for hours on end.
Remixes from Underworld themselves (a la "Barking Mix" of King of Snake) a month or two after the fact. A whole "Reworked" remix album featuring only Rick, Darren, and Karl's changes and additions would be a REALLY interesting idea.
Downloadable song stems and WAVs of individual loops, beats, tracks, vocals, etc. for endless and creative fan mixes/variations. This could be done with a code printed on a ticket where you'd go to a website to download. You could include this ticket in normal edition too and charge for the content or limit it to somthing like "try one song, buy Stems of the rest of the tracks for X amount of $")
Something nice to display at work in a cubicle that's classy and isn't all marketing and advertising "UNDERWORLD!!!1!!!". Just some nice art for people to talk about or admire as they walk by.
Rock Band or Guitar/DJ Hero tracks from this or previous albums.
Numbered and signed drop of blood from all three band members.
Really depends on what's in it for how much I'd be willing to pay......US $40-$50 is about where I'd draw the line though.