First of all, I am not a lawyer. If you want legal advice, find yourself one. What follows is my semi-educated opinion.
‘IP’ isn’t a single legal thing. What people refer to as IP is copyrights, patents, trademarks and trade secrets. These are all quite different things, with very different laws. Many of these laws also vary by region. Copyrights and trademarks are most relevent to your questions.
Copyright prohibits you from using images, sounds etc. from a copyrighted work in your mod. For example, if you were making a simpons mod, you couldn’t use anything captured from the simpsons show in it. If you hand created something that was clearly a direct copy of one of these things, you could also get in trouble. You could however, make a mod which was recognizably set in the simpsons universe without infringing on their copyright. There is also an aspect of copyright called ‘fair use’, which gives you the right to create parodies and some other things. However, the interpretation of fair use is quite varied.
Trademarks would prevent you from using trademarked names or images in your mod. For example, you couldn’t call your mod The Simpsons mod, since “The Simpsons” is a trademark. You could also get in trouble for using likenesses of individual characters etc. For example, having a character called ‘Bart’ who resembled Bart Simpson might be enough to get you in trouble. Trademark owners must defend their trademarks, or they risk losing them.
Unless you have a lot of money to defend yourself, it is much better stay well clear of other peoples IP. Corporations have been known to sue their fans for producing “infringing” fan art, on the weakest of legal grounds. They have also been known to sue completely unrelated projects which used a similar name. Even if you are within your rights, chances are you do not have the resources to prove it in court, and even if you do, by the time the case is settled, your mod will be years out of date.
ET being and your mod being free is not really a factor, IMO. It might reduce damages if you did get in trouble, but it doesn’t give you any additional rights to other peoples IP.
All that said, there have been many mods/character models/maps which were based on the sort of thing you mention. Many have not had any trouble, while others have. In general, it is quite likely you will get a cease and desist letter before your actually get sued. If you promptly comply, chances are good your only loss will be however much work you invested in the project.
You can also try to get permission from the owners of the IP in question. Sensible companies support their fans. Sadly, they are few and far between.