@TigerJoy - you're not Peter Bowker are you? 
However, I always enjoy a bit of light hearted speculation so here goes...
Lois's brother, Tom, last seen entering Spain - sort of neutral with fascists in charge . Seeing as he wasn't very keen on fighting, he may choose not to return to England at all but spend his time loafing around some neutral place - think Lisbon or Casablanca - ducking and diving while trading on the black market like a 1940's Mancunian Del boy.
Lois and Vernon - if he survives the Battle of Britain - marrying but she soon realises that worthy and kind as he is, there's no spark and she'll be looking around for excitement and probably itching to get back on the stage. Perhaps Randy will live up to his name ?There will be trouble.
Douglas and Robina - their friendship will continue to blossom and, despite her misgivings, Robina will become attached to Lois's little girl, thinking that she might be the only grandchild she will ever have. (By the way, has anyone ever explained why Harry can speak so many languages? Being able to speak Polish without having a Polish-speaking parent, seems unlikely in an upper middle-class chap)
Nancy - she'll continue to rub the authorities up the wrong way until they kick her out. She'll have to leave by the end of 1941 anyway. Perhaps, she'll be sent to England by the end of 1940 to broadcast from Blitz-torn London about the plucky Londoners plight, like a female Ed Morrow.
Harry and Kasia - hmm, all depends doesn't it! If she doesn't survive then he'll be back in England, depressed and broken, but still in love with Lois and realising it's too late. He'll probably volunteer for more hazardous missions to try to forget both her and Kasia. If Kasia survives, perhaps they'll end up trekking south through Europe and eventually end up in Cairo, just in time for the desert war to start - memories of 'The Fortunes of War' come to mind.
Paris doctor and his lover - difficult to say. Maybe he will marry the Jewish nurse to save her and think of returning to the USA before Pearl Harbour, or maybe they'll leave it too late and end up in Vichy France or join the resistance.
I think the writer would like a series for every year of the war, but someone else had that ambition for 'The Crimson Field' and it didn't get beyond one series. I just hope whatever they do, it has a natural and distinct end.