IMO the key thing, and it comes out in many of the suggestions others have made, is to predominately have an acidifier, probably lemon or lime, maybe vinegar, maybe a combo. You will also need salt unless the fish has already been well salted. Pickles, for example, meet both criteria. From then on you can do just about whatever floats your boat. Some heat is good. Tartness, as with mustard or horseradish, is also good. As to mayo or other oil-based ingredient, I would go easy on that since the fish may already be fairly oily, but it depends on just how oily it is and your own preferences.
Yellow Pepper-Cilantro Pesto
2 large yellow bell peppers, grilled, peeled, seeded and chopped
1 clove garlic, chopped
2 tablespoons pine nuts
1 cup chopped fresh cilantro leaves
3 tablespoons grated Parmesan
1/2 cup extra-virgin olive oil
1 teaspoon kosher salt
1 teaspoon freshly grated pepper
Blend and slowly add oil to emulsify.
I usually double the recipe and freeze to use as-needed; it keeps amazing well.
How about a salsa verde?
It works on fish tacos so it should work on fish sandwiches.
Poach 4 tomatillos (husked), 1/2 jalepeno (seeded and stemmed), 1 quarter of a yellow onion in water and a spalsh of white wine vinegar until soft
Place in a blender with ground cumin, salt and pepper to taste
Add a couple of good handfulls of cilantro (stemmed) and the juice of 1/2 lime and pulse
Adjust seasoning to your taste
Or maybe an olive relish of some kind.
chop up some canned chipotles, and blend w/ mayo and a little bit of the adobo sauce they come in.
Mayo loaded with wasabi
Pineapple salsa, mango salsa, peach salsa
Rosted Red Pepper Coulis
Olive oil, salt, pepper, a good handful basil and garlic pureed in a blender.
Indian Pickles / chutneys
Tzatziki - fish gyros in a pita. (I do this a lot)
You have plenty of options. Tartar sauce (although I like it) is one of the LAST things I think of for a sauce for fish.