Some of the flavored creamers are pretty good. You can also buy the syrups they use as coffee shops, if you prefer your coffee black. They will add sweetness, though.
At a cake/candy supply store they’ll have flavor oils for candy making. You can also try flavor extracts for baking, available in the grocery story. They won’t impart much flavor without a little sugar and fat to spread it around your mouth, though.
Maybe I’m missing something, but have you tried just plain flavored coffee? Ground coffee beans that you brew, not instant or pre-sweetened. Like this or these. These aren’t sweet, just regular brewed coffee with flavor added, so you would still have to add syrup/sugar/flavored creamer if you wanted sweetness or creaminess.
I don’t know, but I’d try cocoa – the problem with actual chocolate is that it has a bunch of fat which won’t mix into the water very well.
Mix the cocoa right into the coffee (or, make strong coffee, whisk a tablespoon or two of cocoa per cup of coffee into a little hot water until it’s dissolved, and then combine with the strong coffee. Put sugar in with the cocoa if you want)
If you have a Keurig type coffee maker and a large mug you can run a coffee packet through and then a hot cocoa packet through and drink. It worked for me when I tried it as an experiment (and by experiment I mean I accidentally hit the wrong size on the screen and ended up with a half filled mug so I decided to improvise and see how it tasted).
It never occurred to you to buy a bottle of Hershey’s Special Dark chocolate SYRUP and, you know, squirt some in? No need for this dry cocoa powder nonsense. They make liquid dark chocolate for exactly this purpose.