Vanilla - Wonderful taste and aroma. Use pure extract or a vanilla bean husk .
Cardamom - Noted for its fragrance. Use the green variety.
Fennel - A few seeds can be added if you like fennel.
Cinnamon - Add cinnamon in stick form.
Cloves - Commonly used in chai,  these should be used whole and only add a few.
Pepper  -  Add a few peppercorns crushed.
Ginger  - Best to use fresh ginger.
Allspice - Allspice tastes like a combination of pepper, cinnamon, nutmeg and cloves.
Coriander - Roasted and crushed seeds of the cilantro plant.
Chocolate -  Use cocoa power unsweetened.  Top lattes with chocolate shavings.
Licorice root - A little goes a long way.
Anise  -  Ditto.
Nutmeg / Mace  -  Add at the last minute