I agree with Michael - fresh banana is my default additive. But taking a lesson from the smoothie-ologists,
frozen banana is even better as it adds a creamy texture and eliminates Huel’s ‘powdery’ characteristic that some people find objectionable (not me, actually).
This can also be cheap: retailers will often sell off ripe bananas at a heavy discount, so buying in bulk and freezing the lot is an economic solution to enhancing Huel, as well as a natural one.