You could sell a game guide for D3, however Blizzard legal hates those. They've done stuff like file takedown notices with ebay, over and over. Then when the guy filed a counter notice, they refused to go to court, waited for the time period to expire, then started filing more takedowns for his new game guide auctions. This disgusting behavior got his ebay account suspended! And they refused to ever explain specifically how the game guide violates their copyright or trademark (hint: it doesn't.).
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.
It seems to me that usually these game companies have a publishing (as in, books) branch that manages the official guides. THEY'RE the ones who can't accept any competition, possibly due to their contracts with Prima/Brady (or whoever).
Yeah of course it's fair use. A guide is a commentary work and it's not even close to substituting for someone buying the game. The point of copyright law is for the copyright owners to be able to make a profit -- copyright is to help encourage people to make and sell stuff. You aren't competing for sales with their game, so there's basically no way you're violating the game copyright. Just don't include video walkthroughs of the whole game (which could actually be a substitute good for buying the game).
If they've published their own game guide, just don't take any material from it and you won't be violating any copyrights on that either.
For trademark, the point is to prevent consumers from being confused about who is responsible for products, so people can build up a brand name and have a reputation they are responsible for and have control over. Guide titles like, "Ubergeek's Ultimate Unofficial Guide to Diablo 3" do not violate trademark because they do not cause any confusion about whose product this is. It's well established that you can use someone else's trademark as a substring in your product title when it's important to explaining what your product is and won't cause consumer confusion. (Similarly, "Pegasus Case for iPhone" or "Ubergeek's iPod Battery Replacement Kit" would not violate trademark.)
But, yes, as you say: being completely right doesn't prevent you having to defend yourself and spending time and money on that. (Could you recoup lawyers fees after you win? I have no idea about that.)
DISCLAIMER: I am not a lawyer. (But I have read some legal sources about this.)
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.