I've mixed all sorts of stuff in, and even the default rules have various interactions more complex than just ODF/DDF (blunt weapons, scale piercing, etc.). There's a risk of going overboard, but minor codification should be fine.

I usually make it as complicated as my players want it to be. Some of my players come from the pathfinder/5e world and like all the other extra stats to enhance their ODF and DDF while wanting to keep with ease of Fudge. So, I add as much as they want to it.

Immunities to a particular form of damage or attack are always exciting twists to trow at players, whether in Fudge or any game system.

Another method I had considered reduce the match for powerful characters vs powerful opponents was a new use of scale. I had a set of adventures in mind where when the player advanced to a certain point, I was going to introduce a "Heroism" scale that was analogous to collections of levels in D&D. Once minimum pluses to combat essentially meant they would always or nearly always exceed a Legendary result against a normal opponent (of the same "Heroic" scale), they would enter a new scale and against opponents of a similar scale, they would be essentially equivalent to average humans against equivalent foes. I was planning on just applying this by Fudging it and using a trait ladder.

Something like, after the characters had adventured in the local area defeating monsters and gaining a reputation, they would now be "fair" heroes, considered a level lower, or mediocre in the larger nation, and probably "poor" heroes compared to adventurers whose reputation and great deeds spans multiple countries, but then they move on to bigger adventured across wider areas against tougher opponents they become "good" Heroes and so on, adjusting or leveling abilities accordingly

Play never got far enough for me to flesh out the details.