What are you thinking about?

[Encia22]

A little aniseed or star anise would be worth it, I'd imagine a bit syrup will be needed too for fireball, maybe honey or maple syrup just because?

 
Sambuca liqueur is quite dense, so it would have the duo effect of syrupy and aniseed.

https://en.m.wikipedia.org/wiki/Sambuca

In Italy, people use a shot in espresso coffee for sweetness and the hit... it's quite strong. 



Edit to add: Ooh, nearly forgot something else we do with Sambuca:

Serve straight in a shot glass, add three coffee beans.
You can light the sambuca for a nice blue flame (turn out the lights). 
The coffee beans represent Health, Happiness and Prosperity, and are eaten along with the drink...,
Oh, remember to blow out the flame before sipping unless you want to spend the holidays in a hospital E.R.