Something about Herrera scares me. He still throws hard but there's a lot of innings on that arm and he's not a particularly big guy (5'10"). I admittedly haven't done a ton of extensive research on this, but it seems like guys who can sustain throwing 96 into their thirties are a lot bigger. Greg Holland's another guy around the same size who experienced velocity/effectiveness loss a couple years ago.
Some off-the-radar trade candidates I like (making some $ on go-nowhere teams) include David Hernandez (CIN), Andrew Chafin (ARI), Drew VerHagen (DET), Hector Neris (PHI), and obviously Will Smith (SFG). Neris in particular is intriguing. The Phillies are obviously going for it, but they have a ridiculously crowded bullpen (Dominguez, Neshek, Hunter, Alvarez, Ramos, Neris, Nicasio, Morgan, Pazos, Garcia, Davis, Rios, De Los Santos, et al.) that includes a few unmovable contracts. Neris had a longball problem, but a higher swinging strike rate than Josh Hader.