I liken mod tools, mod chips, etc etc to a gun. They are a tool, it is legal to own a gun, it is illegal to shoot someone. The same with my original moded xbox, it made the best media server ever. The second I download a game I don't own and play it, I've committed a copy right violation, but until then, all I've done is mod hardware that I own.
Also, a lot of times EULA's and liscenings agreements and all sorts of things that you sign, may not actually uphold in court (if you've got the cash to prove otherwise). Sort of like in CA I see game developers sort of laugh at non compete clauses they sign since basically you couldn't hold someone to them, since they basically tell a person they will have be unemployed after leaving the company they signed it with, you just cant force a person to do that.
But anyway, Janlulhannes has the spirit of it right there. Look at all the mods out there, every game has like a star wars mod, a lord of the rings mod, a battlestar galatica mod, a warhammer mod, whatever. Only in EXTREMELY rare cases do mod creators have company permission to create the things they do with their IP.
Finally, in terms of derivative works and blizard themself, look at their stuff! Warcraft and Starcraft are disgustingly obviously copies of gamesworkshop's warhammer fantasy and warhammer 40k, everything down to the art style even! It now has a life of its own in bliz world, and there are enough 'differences' but yeah, its pretty much pound for pound imitation of it.
So use that as an example of derivative works and whats 'safe'.
Anyyyway, just go on making what you want to make, post it all over the internet, and that'll be the end of it