isochronous endpoint I/O
optional timerfd support (runtime detection)
API docs
notifications of hotplugged/unplugged devices
thread safety
use poll() rather than select()?
abstraction for cross-platform-ness

API style/naming points to reconsider
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struct libusb_(bulk|control)_transfer or parameters?
devh in general
urbh in general (should this be a transfer handle?)
find/get devices API
config struct/function naming