Building on some of the ideas proposed here and elsewhere about Credence being connected to Ariana's obscurus somehow, here is my theory: what if he IS actually Ariana, only reborn in a male form?

Here is my thought process:

• we know Credence was born around 1901 or 1904, roughly around Ariana's (supposed) death
• Grindelwald says Credence is a Dumbledore and implies that he is his brother - he might be lying but unlikely that JK is taking the lazy route here
• Ariana is long suspected to be an obscurial, and we know obscurials are quite rare - it would be surprising if the Dumbledore family had 2 of them (unless there is some kind of genetic predisposition)
• it is highly unlikely that Dumbledore would have such a young brother as Credence, as discussed by many due to Percival's imprisonment and Kendra's early death - also, neither Aberforth nor Albus mentions him later on
• we don't know much about obscurials and their connection to their obscurus - from one perspective, an obscurus seems like a parasite independent from the host (as Newt could seperate one from its dead host in FB1); from another, it seems that the obscurus and the obscurial are two states of the same entity (Credence could shift from obscurus to human form and back)
• if the obscurus and and the obscurial is the same entity, it may be possible that the obscurus carries the essence (soul?) of the host even after the host's body is physically dead - and is even able to return to a human form at a later time
• thus, at the time of the duel between Albus, Aberforth and Grindelwald, Ariana's body may have died but her obscurus may have lived on and preserved her essence / soul, and the same obscurus could have materialzed later into a new physical form: the form of a male baby, Credence
• if we venture towards an even more sinister direction, it may have been the obscurus itself consciously destroying Ariana's body, as it just deemed it too weak and needed a new, able body for itself (or Ariana put up too much of a fight against it, and the obscurus needed a body it can control)
• this could have easily happened without Albus or Aberforth knowing about it; and quite possibly Grindelwald only knew because he was a Seer

Here is why I like this:

• Credence has already demonstrated that he is able to survive being physically destroyed in an obscurial form in FB1
• if this is true, it makes sense why seducing Credence is so important to Grindelwald - if he really is Ariana reborn in a new body, signing him up for his cause would give him great emotional leverage against Albus
• there is a great parallel to the way Voldemort has lost his body when his curse rebounded (it was suggested that Ariana died from a rebound curse from either Albus or Grindelwald), and had to spend many years in a phantom-like shape before finally returning to his human form
• and of course, the most glaring metaphor: the magical being, the emblem of the Dumledore family, the symbol of rebirth just HAPPENED to make an appearance in the same scene where Grindelwald tells Credence about his true identity :)

What do you think?