Only thing is a $9.99 megacomic won't work if you want color. People want color with their comics, especially western ones. Manga gets a pass because it's always been black and white, but there's a reason why Marvel and DC and the others don't do B&W comics now (I don't count the Showcase and Marvel Essential phone books). Yeah, you find guys like Dave Sim doing fine in B&W but they're the oddity. The UK used to do this a lot with their weekly titles that reprinted Marvel US comics in B&W, but people complained and wanted the color originals, which they eventually got.
Having said all that, the 120 pages proposed is only 20 pages more than what DC was doing regularly in the 1970s and 80s anyway with titles like Superman Family and Batman Family, and for a time they went omnibus with others as well. And they were full color.
But Leroy's comment is valid too - if you're only interested in Power Girl, say, would you be willing to pay 10 bucks for a thick magazine that contained only a few pages of her story and the rest featuring characters you have no interest in?
Alex