While I'm very (very) excited for this game.. I'm very nervous about my productivity this summer. There are few things that can pull me away from coding. This is one of them. Maybe D3 will open some new startup opportunities? Yes.. yes that's what I'll tell myself. It's all research.
I personally wanted to make a game guide for Infinity Blade 2 but epic legal threatened me out of it (and we had a friendly pre-existing relationship, which counted for exactly nothing. they'd interviewed me for their website and sent me free stuff, and i had a free guide on their forum with 200k views that they had made sticky). Dealing with their legal crap wasn't worth the hassle. I was even willing to give them undeserved money to be left alone, but they wanted a large cash payment upfront and were unwilling to discuss any other payment approach.
Anyway the point is game guides should be a pretty good small business opportunity -- there is a very real market for help playing games better and being more successful in them -- but there are (baseless but real) legal problems so be wary.
You might expect game companies to be thrilled to have services sold around their games, which their customers find provides game-related value to them. Just like Apple is happy to see other people selling iPhone cases. But a lot of game companies are not happy and will harass you.