Let's say I make a collection of house rules for one of my favorite tabletop games.   (e.g. like this)  Kind of like a mod or unofficial patch but for tabletop games.  Will that be allowed or no? 

E: I think the word for it is Homebrew because it's not an entire system just some changes to an existing one.

Of course it will! After all, what you do is what you want to do, and "mods" for tabletop games are a great idea :D
Have fun working on it!