I too believe, all things considered, that the final character will be a first party character (also could be one of the reasons we are going in basically blind).
I'd love it to be Isaac, taking into account the second chances theme of this pass, the results from the ballot, everything that happen in 2018 from the Rathalos Tweet to the outcry when his AT was shown. I'd even think using Dad Isaac, Felix or Matthew could solve the "but he is already an AT!" dilemma.
That said, I don't expect it to be Isaac, mainly because Sakurai just never talks about him, he is one of few big first party character from the ballot that never gets addressed or discussed at all in the columns or other commentary, unlike the rest of the first party characters that got their second chance in this pass...
Hope definitely dies last tho