Roughly in order of importance:

* Add docstrings.

* Implement indexing of sub-arrays (i.e. a[0] for a 2d-array), perhaps even slicing.

* Implement missing arithmetic operations.

* Implement missing comparisons.

* 0-d arrays should not be sequences.  Currently there are problems with using
  0-d arrays with PySequence_Fast.

* Merge array_from_arraylike and matrix_from_arraylike.

* Optimize readin_arraylike for elements that are tinyarrays.

* Implement concatenate.
