In reverse order of likelihood.
Schalke - I do like them and they have a strong history of having American players. Great stadium, and a massive fan base. A big sleeping giant for sure.
Freiburg - So incredibly likable the more I read about them. And honestly, they have some players that I really like (Til for one).
Augsburg - Bavarian, which is where most of my beer comes from. Very cool looking stadium, kind of under the radar, and honestly, I kind of like that they don't seem to have fans on here. I watched their match against Celle and got really into it. Also, I love their kits, my favorite colors are red and a darker green, so that's brilliant.
I was thinking Union, as well, but I like them, but don't love them.