Small spoiler warning (I tried to avoid them): I honestly doubt there will be a sequel but I still hope there is!
My theory of what happened to Uncle Jack is that he is Foggy Jack. In the broadcast room, he seemed so distraught after lying to the citizens of Wellington Wells and lying to himself for 17 years. As soon as he was suffering from withdrawal from joy (seen during one of his last tapes where he revealed that Ollie was the reason Margret is.. in his head (trying to avoid major spoilers)) he remembered her and couldn't take it anymore leading to a mental breakdown. He stormed out of frame, grabbing a cricket bat in the process, then proceeded to destroy the camera that was taping him, all while chanting "we've come to the end of our time!" This is the phrase that was found on many of the dead bodies in notes.
As far as we know Percy is in Germany -- but that was 17 years ago.. (The train Percy was forced onto took the children to Germany.) I'm dying to know whether Arthur actually finds Percy or not. The developers may have done a game design choice where the players basically get to think whatever they want at the end of the story. Whether Arthur found Percy, well that's up to the players to decide. If the developers actually do have an idea it also depends on which ending in canon (true to the storyline). Did Arthur actually choose to remember Percy and walk across that bridge to the outside world in hopes of finding him? Did he choose to forget after all that and face the reality of possibly never finding Percy? Or did he simply choose to forget way back in his office when he first saw Percy in the newspaper article?
I honestly have nothing for Sally. I mean she probably had the best ending. She escaped with Gwen and probably just travelled somewhere relatively close like Germany or Spain to live the rest of her life and making it the best for Gwen. Everything she did was for Gwen-there literally wasn't a single moment in the story missions that were not to get Gwen out of Wellington Wells. Hopefully, she wasn't underestimated as a woman in the 1960s and 70s (or anytime for that matter) and got a career in the chemical aspect of science.
As for Ollie, he probably also went to Germany or Spain or maybe he went home to Scottland? It seems he's trying to move on from Margret rather quickly (as seen by him dropping her picture while still in the float) so hopefully he decides to do something with his life and maybe do more service if that's still an occupation in this world (I'm pretty sure Germany won the war in this). Even though it may look like he's moving on, Margret will probably forever be in his mind and hopefully, he'll try to make her proud. She told him to put away childish things so I'd like to think that (after pissing over Wellington Wells) he became someone great. Because "[she's] right, [she's] always right."
Did anyone also notice that when saying goodbye to Ollie, Margret sounded Scottish? She picked up on his accent because if you're around him, a man with a strong, STRONG, Scottish accent, you'll probably start to say things that way too.