If you want a snake that's arboreal and slightly nippy, I'm going to again nominate beauty snakes. Depending on the species they can be a bit large (VBB get around 9' but can get closer to 12', and Taiwans usually get around 7'. Chinese usually get 5' I think) but they're also very slender in comparison to their length.
From what I've read if they're not handled regularly they will be more defensive and possibly bity. They're highly aboreal as well. And also beautiful, but I might just be biased lol.
(as a random wild suggestion check out baron's racers, they look so interesting!)