Oh boy, where do I begin? (We love risotto around here.)
I agree wholeheartedly that the broth you use is everything. If it is good quality, as are the butter and cheese, you don't even need to add anything! 
Another citrusy one is leek risotto with lemon zest and scented with rosemary.
Another delicious one is radicchio. Can also be finished with some sort of white cheese.
Risotto with shrimp (Can be with tomato or with zucchini or with saffron.)
Risotto with calamari
Risotto with mixed seafood (clams, mussels, shrimp calamari)
Risotto with saffron is delicious with pieces of sausage. Instead of Milanese style it's called Monzese (a city outside Milan).
Risotto with pear and pecorino (or gorgonzola or taleggio) is popular here (but I don't like it).
Spring Risotto with new spring veg
Truffle risotto
Asparagus risotto
