What song should I use as a base for a fan fic?

I really want to make a fan fiction for rin and len kagamine based off one of their songs, but I just can't make up my mind. I kind of one to do a rinxlen thing without twincest, but I don't really care if I do. So can anyone give me a good list of their songs. I know a lot of them but everytime I try to think up one I just blank. Please help and thank you!