Sounds like tuna salad, which is delicious. Add spring onion or red onion or whatever oniony thing you want. Possibly a bit of celery if you're into that (small pieces so it's not stringy).
Try some citrus zest and/or juice. The juice is only if the rice isn't too acidic already.
Oh, and if you're into spice, sriracha goes on a lot of different things - it is chili sauce with vinegar and garlic, which seems like a good fit. If that isn't your thing, you can still bring the spice level up with cayenne or whatever chili flake. "Chili powder" could work but often has some un-onigiri-like extra spices.