I love Jimmy John's veggie sandwiches, and what they put in them would also probably work just fine in a wrap: Guacamole, tomatoes, lettuce, bean sprouts, provolone cheese, mayo, and cucumber - but I also add their optional roasted hot peppers too. They took the bean sprouts off the menu about a year ago, due to some bad batch which was contaminated, and haven't added them back since unfortunately - but of course if you are making them yourself you can.
There are numerous recipes out there for a tuna and white bean salad, which also works well in a wrap. It's a nice change of pace from the usual mayo and tuna - instead using vinegar and oil. Plus the white beans add extra bulk and fiber.
Honestly, I could just be fine loading it up with some roasted and marinated bell peppers, as well as marinated mushrooms and lettuce and cheese.
Or perhaps a turkey rueben - turkey, coleslaw, thousand island dressing, & swiss.