Some things that need to be done in no particular order:
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Implement isochronous transfer mode.

Implement audio driver.

Do bandwidth accounting.

Use lock manager locks.

Use the pool allocator for TDs et al.

Merge soft_td and td in uhci.c.
      soft_qh and qh
      soft_ed and ed in ohci.c
      soft_td and td

Tidy up transfer completion.

Throw out more USBDI gunk.

Port FreeBSD umass driver.

Make hub driver note transition between self and bus powered state.
Handle overcurrent conditions.