Watch the Advanced Left-Hand Demo online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
Here are some fun and wacky variations to liven up your fills. Instead of just playing quads and triplets, try playing 5s or 7s or even 9s.
You don't need to play all 5 or 7 or 9. For example, instead of playing all 5, just play 1 and 3 - it sounds like a short beat and a longer one. Or instead of all 7 play 1-3-5. I call these patterns "codes".
Try it yourself. Start with playing all the hits and experiment with leaving some out to form your own codes.