The Doom God wrote:
"... a ?mod? or any similar project cannot be based on intellectual property belonging to any other person, company, or group without permission. You cannot make a mod based on the ?ALIEN? universe, the Simpsons cartoon show, the Terminator Universe, the Mario series of games, and so on. Those characters and universes are the property of their respective companies, and they have the right to decide when and where they are used (barring news coverage and similar ?fair use? cases, which do not apply here."
Let's clarify something here.
"Cannot" is simply not correct. As we have seen, it is, indeed possible. The simple existence of AliensTC, the various "Aliens vs Terminator vs Predator" files, the "Doctor Who", the "Star Trek" and the Simpsons files available in Ty Haldeman's Doom archive should prove that it is, indeed, possible to do this sort of thing.
"May not legally" is the accurate way of phrasing it.
This is NOT just playing with words. Accurate phrasing is crucial to ANY activity relating to legal matters, which is where this discussion has gone.
Companies like id software and 20th Century Fox (for example) have recourse to legal action if they consider that someone hase used their intellectual property in a manner thay don't approve of.
Now, one of the legal requirements for intellectual property rights is that owners of such rights MUST enforce said ownership.
A real life example, now. Anyone here heard of the term "aspirin"? Thought so. What many of you won't know is that aspirin was, originally, the trade name of the first commercial preparation of "acetate of salycylic acid", and the first commercial analgesic preparation. Unfortunately for the owners of the name, everyone, including their competition, started using "aspirin", to the point where, in public awareness, aspirin was now associated with the product in general, rather than with a specific product.
Xerox is another example. The Xerox Corporation failed to rigorously prosecute violations of its trademark name, and so, now (at least in America) "xerox" is the general term used a photocopy. Courts have now ruled that Xerox Corp. can no longer sue other bodies for this particular use of their name, because Xerox had allowed the term to enter into general use.
"The companies being "robbed" of their intellectual property have no reason whatsoever to press charges as long as they're not losing any money."
I'm sorry, but this simply is not true. The subject goes far beyond the issue of "losing money".
If copyright holders don't aggressively protect their rights, they run the risk of losing them.
Back to TC and mod-makers.
There is nothing, physically, stopping you from making the TC or mod that you desire. You must, however, make a decision on the legal status of the project.
If you decide that you want permission, then you must face the possibility of refusal. Should you fail to get permission, you must then decide if you're going to abide by that failure, or proceed anyway. The risk of proceeding is, of course, that you cannot plead ignorance should the copyright holder decide to pursue you.
Some years back I ask Ty Haldeman, maintainer of the Doom archive, about the status of a mod I wanted to make, as it would have incorporated graphics from another game. His response then (I regret that I can't find it) was to the effect that anything that I made could be stored in the archive (then cdrom.com) until and unless the copyright holder complained about it.
What does this all boil down to?
Simply this: go ahead and make your TC / mod if you want, but be aware that the owner may come looking for you.