Back when I went to college (the last millenium!) we had to pick a major out of the catalog, not create one. I guess professors were suspicious of what we'd make up.
Anyway, if you want to use fragrance to improve mental health you'll need a combination of chemistry, biology and psychology. It would probably be a psychology major with a minor in chemistry (so you can make your own) and some supporting work (maybe a self-study course) in olfactory receptors and their connections to the emotional and cognitive centers of the brain.
Maybe your first essay should track down any scientific papers that might have expored the efficacy and methods of aromatherapy. I started a thread on this, asking people for their experiences, and there was very little response. Some people suggested books or web sites that assert the value of aromatherapy. Others said that smelling a nice 'fume makes people happy and happy people heal better. You probably need more than that before you decide to invest 5-7 years.
Good luck & please post your experiences here.