To Do (mainly in order of priority):

Search and destroy the existing bugs.

Fix "randomness" of flags; particularly Sign of Bit 7,s

testreq at GoLowBuf (?)

Undefined-opcode exception, and other possible uses of a general
exception handler call.

Optimisations for higher processors.

Better documentation on how to write instruction routines,
  including full descriptions of the implementation-dependent
  instructions (flags and such).

User patching of instructions.

Consider return upon Halt, pulling Z80 HALT line low.
  Would that be a general exception, or a separate thing?

Implement interrupt mode 0? How then?

DMA-request to make sure external write to memory is between instructions?
  (Simply use Exit? Slow, but...) This also a general exception?