i kinda do different if making it melty, or just a sammich.
Both would have some sort of minced chile or sambal olek, some kind of pepper powder, sweet and/or dill pickle, chopped onion, seafood seasoning (it's basically celery salt w/ spices), lemon pepper, garlic powder, stone ground or chinese mustard, chopped olives (green or black depending on mood) and Mayo.
My Korean chili paste works well...remember..I've got tins of tuna with birdseye chilis IN THEM already...it's a natural.
Bread is important too..generic white bread is only good for sticking on walls....of yer colon.
I prefer french or italian bread for tuna, but multigrain is good too.
Main diff is the consistancy and cheeses.
I like it pretty mayo-ey as a sammich and with a nice sharp cheddar. Melted, it'd need to be more solid and likely using pepperjack/chipotle cheddar.
I usually chuck in a splooge or two of hot sauce too, just depends.
Another thing that's great with tuna: Curry.