Some open questions:
Full treatment of Zeitbewegung, how to include all paths in the analysis, not just those that cross once.
We have modeled a calm cat; we need to be able to model a nervous one.
Treatment of multi-particle case, i.e. quantum electrodynamics.
Needed for reasonable treatment of photon electron interactions and the like.
Implications of full time/space symmetry for time/energy uncertainty relation
For instance, Aharonov and Rohrlich note that the Aharonov-Bohm effect needed to ensure consistency of quantum mechanics; presumably the same statement true in time.
Implications of temporal quantization for duality: flipping of and x induces flip of electric and magnetic fields.
Implications of laboratory time for measurement problem