would a good Mozzarella and (insert other cheese of choice) not be the a good combination to try? Grated mix of course, as you get the stringy-ness of the Mozzarella with the awesome flavour of whatever other cheese you pick?
That's why I suggested monterey jack and cheddar.
Monterey jack is stringy like mozzarella, but had a better flavour. Add some mature cheddar to the mix and you have a recipe for win.