Everything in your spice rack is a possibility
I’ve used a blend called “pumpkin pie spice” and “apple pie spice” and whatever curry blend you have works too (if you like curry)
I’ve seen Root Beer extract (like vanilla extract) but haven’t bought it yet for in my Huel. There are lots of store-bought mixins for coffee; either flavored syrups or creamers.
I honestly don’t really try that much to flavor Huel - it tastes pretty good with a little juice and water, so mostly I only mess around with something different on weekends when I have a little more time to think about it.