Some things that need to be done in no particular order:

Implement request timeout and abort in ohci.c.

Implement request abort in uhci.c.

Use the new device attach/detach framework for USB.

Implement isochronous transfer mode.

Implement audio driver.

Do bandwidth accounting.

Use lock manager locks.

Implement a modem driver for the Abstract Control Model.

Implement USB suspend/resume for APM suspend/resume.