SEN-DUS was operated back when there were no three daily flights each from STN and LCY...
DUS has plenty of flights from LHR, LGW, LCY and STN. Don't think there is room for a once daily SEN service.
I think Flybe will need to look for a third-level airport like PAD, SCN or ERF without a LON service and not a major airport for which the SEN service would be like the 20th daily London flight.