The Perfect Toastie  
Some of the best combinations:
Cheese & onion & kasundi (Indian tomato chutney)  
Sausage, mustard & edam  
Tomato, basil & parmesan 
(my childhood fave was always creamed corn, cheese & onion)   
Tips for the perfect toastie: 

The bread; for once I'm not going to advocate for using a quality ingredient because frankly, thick white toast bread will still produce a great toastie! But a sourdough will add a tang and good texture and a rye could offer up a great flavour. Avoid bread that has big holes in it, like ciabatta.  
Butter or mayo; Mayonnaise is full of oil so will give you a golden crust and it's less likely to burn than if you butter the bread. However butter does have the edge when it comes to flavour in my view. So, both are good.  
Fillings: anything goes but consider the soggy factor and more is not necessarily better. Try to have a balance of dry (meats, beans, onions) vs wet (chutneys, tomato, cheese, pickles) ingredients 
The cut; the cooked toastie should always cut on the diagonal in my opinion a. to reveal the filling and b. there's something about tucking the sharp corner into your mouth for that first bite.