I'd use panini fillings. At the cafe I used to work at we did:
Egg mayo
Prawn with marie-rose sauce
Pastrami, salami and tomato
Beef, tomato and mustard
Turkey, brie and cranberry
Cajun chicken
mozzarella, tomato, pesto and basil
Bacon, egg, and tomato (or sausage)
Other favourites are...
beans and cheese
cheese and marmite/vegemite
You can make a toastie out of pretty much anything you can think of, when people got bored of their usual lunch they'd be like "I'll leave it to your imagination" so I got to throw whatever I wanted on it! As long as you avoid salad type things you can get away with throwing all sorts on, heh.