Try to avoid infringing on IPs, especially the ones owned by very protective companies. On the other hand, the jam games should be similar enough so you can tell what game was redesigned. The only thing to watch out for is Shadow of Mordor's nemesis system, Warner Bro's holds a patent to it so you can't use it without paying them. (typical 😒)
Actually, there are many game mechanics which are patented, not just the nemesis system. (NOTE: I've taught classes on game business & law, but nothing in this post constitutes legal advice and I am NOT a lawyer.)
For example, scrolling notes in rhythm games (both horizontally and vertically) was patented. It's why Guitar Hero choose to use the 3D "into the screen" view - to avoid infringement.
The truth is that some companies are simply more likely to enforce their patents than others - and it is unlikely (though not impossible) they will bother to go after a free game in a free game jam. #NotLegalAdvice #NotALawyer
(NOTE: There are "patent troll" firms that chase after everyone they can with over-broad patents and attempt to squeeze money out of companies. They *can* go after individuals, but most will prefer to target businesses instead. That doesn't mean you are in the clear, it just means you could get lucky.)
Now, this is VERY different from violating someone's Copyrighted IP (characters, art, names, logos, sounds, etc). Some companies (*cough*Nintendo*cough*) are VERY litigious when it comes to protecting copyrights.
So, what does this mean for your game? Let's look at an example (Again, #NotLegalAdvice #NotALawyer):
Let's say you wanted to remake the original FF7 for the Atari2600. You CAN take "inspiration" from the concepts and game mechanics.
Random encounters? Yes.
Similar enemies and power arrangements? Yup.
Exploring themes from the overall plot? You bet!
A hot-tub scene with "jokes" about sexual assault? Um, please don't, but...technically, yes.
BUT - name your main character "Cloud", use their music, or have your character look like him? Now one of my industry friends has to send you a very, VERY unhappy letter. =(