Filling for Stuffed Chicken Breast
There are a few classics:

Chicken Cordon Blue - Ham and swiss

Chicken Kiev - tarragon, garlic, parsley, butter, etc.

You can use spinach and cream cheese, spinach and bacon, spinach and ricotta.  Mushrooms with different cheeses, there are a ton of different things.

Try this:

google.com

and search:

different types of stuffed chicken breast