Coffee, STRONG BLACK COFFEE!
For giggles one night out at deer camp, we injected a brisket with almost a full pot of coffee. Some alcohol may have been involved in making that decision..
We then seasoned it up, let it rest in the frig overnight, and then put it on the smoker the next day to cook. I'll have to admit, it turned out pretty good, the rest of the folks had no clue what the flavor was, and we've done several since then.
Now, it doesn't taste like a normal brisket, I really can't explain the flavor, but it's pretty good.
With brisket prices as high as they are, I don't recommend trying this on a full packer, but try it on a cheap roast and then smoke it.