If you added a further main ingredient it wouldn't be a cheese & potato pie any more. Seasoning like cracked pepper maybe ? Hot spices ? Your favourite herbs possibly. Perhaps you can get away with adding (lots & lots of) garlic and still call it a cheese & potato pie. Apart from that there are other veggies, such as aubergine you could slice in before baking. Onion maybe.
Alternatively you could make it more exciting by serving it in the nuddie.