Approaching the issue from another angle, i would go for a combination of system selections (including interaction or possible interference with other systems) and characteristics.
A question that comes to mind would then be: which (sub) system components could (due to a yet undefined fault) migrate from commanded position without generating a fault indication to the flight crew but result in reduced fuel flow and cause engine rollback?
Just to be a tad more inclusive in this postulation, also add crew actions as a contributing/causal factor in some way that is not annunciated (time captured) on the recorders. Remember that some recorders don't latch everytime a bit or byte changes.