Try getting ideas from Google Patents.
RFID tagging stuff is just way too simple.
The triangulating idea is somewhat better. If it was fast enough, you could use it in conjunction with sports to improve/record form. You can't do that with a cell phone. Add in some neural network coding in order to find what form gave you your best results, pro.
Or, above triangulation in combination with some other arts. I guess the only thing that applies is dance.