I've done quite a few injections in a pork butt as well. Each is a little different, and usually has some rub mixed in.I've played with so many rubs I've forgotten a lot of them!
Some of the biggest flavor changes have been injecting Buffalo Sauce or Hot Sauce, but I've grown fond of just enhancing the flavor of the meat and not changing it.
Kosher Salt and Coarse Ground Black Pepper are my go to for a lot of things. With that being said, my Vampire Pork is out of this world to me, but most people are afraid of it! Haha.
Ironically, I'm currently working on a couple experiments this weekend to amp up the flavors.
Apple juice
Apple cider vinegar (or a mix of both)
Apple sauce (doesn't leak out)
Chicken broth
Grape jelly (liquified in microwave)
Tequila & margarita mix (really liked that one for tacos)