I always inject turkeys because they’re so mild-flavoured.
My two go-to injections and rubs are detailed in an old smoking book called Smoke & Spice. It was my first smoking book 25+ years ago but I still like it best. One of the injection-rub combo’s is garlic-based, and one is jalapeño. They’re both great, and often I’ll smoke two breasts instead of one whole turkey so I can do both recipes and offer folks a choice (plus the breasts finish more quickly).
My only other injection-rub combo is a spicy one I developed from a recipe Martha Stewart created years ago for Bryant Gumbel of the Today show. You might be able to pull it up on-line.
One other S&S recipe that has become a staple at our Thanksgivings is a jalapeach sauce. It’s rare to find something both sweet and spicy, and folks seem to really love it in place of gravy over the turkey. And it’s very easy to make.