Okay guys, this is just a short list of games and companies its best not to try and make a mod of as if the company finds it, they will serve a cease and desist order - which often happens just before the game happens to be ready for release and its absolutely horrendous to see all the work go down the drain.
It doesn't matter whether its a mod, game (paid or free) you can still get hit with these kind of orders.
(Note - this is a carbon copy of the one I've got posted on Crymod)
SHORTLIST OF IP'S NOT TO TOUCH
Mods based upon ANY Marvel franchise - Marvel doesn't take too kindly to their IP being used, even if its only slightly similar to their work.
Alien/Predator mods - a big no no. Alien VS Predator, is fine apparently, but not one of the single franchises by itself. Strange, I know, but thats how it is.
Square Enix - Square is an absolute git for this sort of thing. They shut down a very, very promising Chrono Trigger mod that turned into a game in its very late stages, just before it were released.
Sony - Like Square, they're the same. Do it at your peril, as they really don't like people using their ideas without permission.
Konami - Castlevania mods have been shut down, and there was one in particular that was thought to be better than the original game, so it got a C&D order pretty sharpish.
Mods based on any Hasbro franchise - Several GI Joe mods have been shut down on other engines in the past, so they'll doubtless do the same to any others that they come across.
The main problem is that companies can perceive a modification on their Intellectual Property to be harmful to them - it may harm their sales, or may harm their appearance as two examples, hence why some companies won't let you do it at all.
3 GOOD RULES TO FOLLOW:
1) Most companies don't like clones made of their work on another engine - particularly if they have the possibility to be better than their own work.
2) You're better off asking permission if it is using someones IP, than not - perhaps they will say yes to you using their work, if you present it as not showing any kind of threat to their work, or a cause of loss of income to their work.
3)You're better off coming up with your own idea, and if it is based upon someone elses company, have it as loosely based as possible - the looser it is, the less likely you are to get a C&D order from someone.
The primary reason for such rules is that they have the legal responsibility to protect the integrity of their IP. Also if there already is a popular game that is a equivalent it is likely to get CD'd (Cease and Desist.) For example a Halo RTS using SAGE engine (command and conquer) would probably get a CD because of Halo Wars (despite the fact that halo wars is a console exclusive. Or a direct clone of Halo (even if its a TC) in Crysis since its then a direct competitor (and not published by microsoft so theres another check against it.) You must carefully think about the reasons why your mod could be canned.
RULES FOR MICROSOFT IP USAGE:
Heres what Microsoft says in a nutshell:
"Here's the magic words from our lawyers: so long as you respect these rules, Microsoft grants you a personal, non-exclusive, non-transferable license to use and display Game Content and to create derivative works based upon Game Content, strictly for noncommercial and personal use. We can revoke this limited use license at any time and for any reason."
RULES FOR GAMES WORKSHOP IP USAGE:
Like Microsoft, Games Workshop is more than happy to let people use its IP, as long as its rules are followed - disobey them at your peril! You can find them here:
Hopefully someone finds some use out of what I've put there!