I HAVE A THEORY THAT MAKES TOTAL SENSE.
Susan Strong is Fionna. The gender bent world is set after the Mushroom War but before Simon and Marcy. Fionna and Marshall had two kids. Fionna fell off the balcony in their house, and was swept away underground. Cake found her but they couldn't get out, so they stayed with the Hyoomen tribe. After Fionna disappeared, something separated Marcy from her dad. BTW Hunsen Abadeer is Marshall in the future. So the thing forced Marcy and and her younger brother from Marshall, and then they got separated from each other. Then something froze her brother, preserving him for hundreds of years. Her brother? Finn. Marshall thought that Fionna had died, so he never mentions her or remarries. It also explains why he is so upset when he Marcy doesn't think he cares about her, because she is the only thing left that ties him to Fionna.
While Fionna and Cake were trapped underground Fionna forgot speech through lack of use, and lost her hat. So Cake posed as her hat instead, but eventually died. Fionna either not noticing (coz she's a little crazy at this point) or keeping her carcass there for company. Ooo is somewhere near Scandinavia, which is where Simon bought the crown. Since the Ice Queen had it before him, her memories of Fionna and Cake are inside his head, which will explain why he thinks F & C are"out there somewhere", even though there's no proof at that point that they exist.
And Gumball could be princess Bubblegums unle and LSP is LSP's dad😄
