As mentioned above, Binders have a great 5th wheel role: if you're willing to the legwork (and note, there's a lot of it), they can take on various different roles, but they also don't shine in them. That is, they won't overshadow anyone.
The OP gives us very little detail about the campaign -- level? Expected level? Party make-up? -- so there's not a ton to go on. I'd second other classes with unique abilities, like the Fiend of Possession mentioned above.
Support characters also seem like they fit the 5th Wheel model: they can't do as much on their own, but they can really help out the party. The classic one is the Bard, which is attractive b/c it also drags in all the fact/espionage stuff, which often might be neglected in a party. I am also partial to the War Weaver -- it's support abilities don't have to just be a bucket of numbers, but can feel like real game changers.