Sure, most video game mods aren't things we can spork, but I'd argue they're, generally speaking, fanproducts (which is a word I just made up to generalize fanfic, fatart, and that sort of thing).

To back up a bit, I'd say what a fanfic is almost always meant to do is to expand on or improve the original cool thing to make it cooler. There's a lot of fic that basically adds more material to the universe, and a lot of the other stuff (AUs, crossovers, and the like) goes "What if we took the thing and made X happen instead?" or "The thing was cool, but I didn't like this bit, I'm writing my version of it instead.". Fanart also expands or improves on on the thing it's being draws about (more art is better, or, in some cases, the original art wasn't good, so it needs to be improved on).

This general theory of what a fan-thing does extends to video games rather well. Quest mods of the sort you mentioned fit well into this framework (there's not enough content about X/the context about X sucks, let's add/replace it!). Heck, even UI mods could be consider fan-UX (this game is cool, but the [whatever] UI sucks, I'm making a better one).

However, not everything fans make can be sporked. Sure, you can concrit anything, but you can't apply the PPC mission format to all media. There's a few prerequisites, like the medium having a concept of plot and characters, and being quotable or summarizable in a way that lets you write a mission about it. (Come to think of it, we could probably spork a quest mod if we really wanted to.)

Now, as to romhacks, I did a few seconds of research. If you're hacking ROM to adjust the game or add stuff to it (like more content or a translation), it's basically a mod that has a particular installation method, and is therefore a fanproduct. Ripping out everything but the engine and building a new game, on the other hand, is roughly equivalent to borrowing someone's basic premise and writing your own version, and doesn't count as a fanproduct.

To actually reach the original question, I'd say that mods and romhacks that act on the narrative stuff like content are effectively fanfic (and, analogously, texture mods are effectively favart), and that some romhacks are mods.

- Tomash