I second the "searchable database" suggestion. Complete database. All spells, feats et cetera from all official books. The fact that this hasn't been done already, even though we're years into the so called information age, is incomprehensible to me. Having to bring 4+ hardcover books to a gaming session just to be able to look up your spells? Plus, you'll probably want extra sheets that points out which spells are in what books. Screw that.
So yeah, a database. And it must be a downloadable, stand-alone application for offline use.
"Hey," some of you might say, "that's not possible! If they released something like that people would just download the application and then quit the subscription!"
Fine, but there are ways around that. I'm no software engineer, but it must be possible to implement a function so that in order for the database software not to stop working, it must be checked with a wotc server once ever other day or so. Thus, you will still have to pay up every month or whenever, but you can still copy the database to a laptop lacking internet connection, bring said laptop to the basement of your FLGS or wherever on sunday night and.