APL+Win has few primitive operators, and I'd like to see more created. Compose is pretty important if we can't elsewise have in-line functional expressions. Dyalog's Commute and Power operators should be relatively easy to implement. I can envision that some of their more advanced operators (such as Key or Stencil) could be quite beneficial in APL64 as well.
I would also like to see an explicit operator for running a function in a background thread, but that may be difficult to implement as there are issues that need to be handled when running multiple workspace tasks simultaneously. For instance, the state indicator would need to be significantly revamped to handle simultaneous threads of execution.