Cherry Cream fillings please! And Ginger cream fillings! Milk chocolate is mostly the best but plain chocolate on soft nougat is lush. (Lovells dark Chocolate covered Nougat!!! Mmmmmmmm).
Toffee and Mallow makes a good filling. My ideal bar would be a strip of chewy (but not hard) toffee with a thick Malteeser-like layer on top, covered with chewy mallow and milk chocolate.
Peanuts and hazelnuts are no go for me. And Brazils cos I'm allergic to them. Any kind of creamy, soft, fondant filling is good as long as it is not too sweet. Violet and Rose creams are a favourite so a bar with alternate cubes of this topped by those crunchy violet pieces would be bliss.
Good luck with your project.