My advice...........be brave and individual. I had a matt black union jack on my gloss black mini. Totally original design. Looks like it frost has settled on my car in the union jack shape. Very subtle and very cool. Never seen another one like it and loads of people have told me it looks great. Don't just go for something standard. Take a chance. You can always remove it if you change your mind. I have also gone a bit crazy on my mirror caps. Not quite as subtle as the roof but just good fun!!!!
Just go with your instincts and have fun designing!!!!!
for inspiration. He is a top guy and can help with any design