PAD used to have links to STN, LCY and MAN in the past by AB and WX. Because it is the heartland of the British Rhine Army, there are still reasonable links between the region and the UK.
KSF is a new airport desperate for airlines. It is halfway between DTM, HAJ and FRA. Relatively little demand in the immediate city, but a fairly large catchment area.
SCN was in the running for a SEN by OLT Express. SCN has been linked to a LON service a couple of times in the past, so apparently someone sees a market. The catchment area also goes into parts of Lorraine.
I have some doubts about LGG because it would have been the obvious alternative to MST but lost against it. So if MST does not work, can LGG?
OST - maybe. But Ryanair was unable to make it work.
EBJ: I mentioned this one because it was a Maersk Air route from SEN in the late 1970s/early 1980s (with Dash 7, IIRC). I don't think Odense would be a more obvious - no schedules at ODE for quite some time because of the improved ground transport links to Copenhagen. EBJ had FR in the past, but might be bit tricky because of Sun Air down the road on BLL-LCY.
IMHO, airports in northern France will have little chance against the Eurostar.
In general, problem with most of the airports mentioned is that they would be considerably further away from SEN than those six currently served - at the moment FMO is the furthest away with 302mi. ODE, for example, would be almost 500mi, AGH 600mi. Too far away for a turboprop that needs to do four daily rotations.