I get wonderfully fresh tuna (maguro) sashimi from a weekend farmer's market that hasn't caused me food poisoning yet, but I don't know if that's already slightly seasoned. The main thing, when I make it into sushi, is adding just a bit of vinegar to the rice. Then, get a sushi roller, layer the damp nori (seaweed sheet), spread the vinegar rice, lay the strip of raw tuna there, and roll it up! Mayonnaise and shredded carrots can be included to make a sort of California maki.
With any fish, I say you can't go wrong with a simple lemon or garlic, with salt. It sounds like you want much more of a kick there, though, and I frankly don't know any other marinades. I'd suggest Spanish seafood paella, but I'm not that experienced with that (requires saffron...which is pricey, and that I think makes paella rather unfriendly and unforgiving to upstart/experimental cooks like us.)
Tuna has a similar texture to salmon, although a milder flavor, so perhaps it would pair well with a spiced avocado puree. Dill is the go-to herb for salmon, but I'm not sure what goes best with tuna only because tuna goes well with a great many.