I guess I'm not the only one suspect that he is a synthetic human

None of the above.

Joe Smoe who lived prewar, no connection to anyone before the bombs dropped.

You could roleplay that he's an ancestor of one of other main characters. Maybe James and Wanderer are descendants of one of Sole Survivor's cousins who managed to survive the war and the aftermath? Though canon-wise, I honestly doubt that.

I really really hope it's not the bad 'you are an android' storyline... I want to play a normal human. Not be forced to be a synth.

The last 3 option are totaly imposible, cause he wakes up from is sleepy time way to late to be annyones father. (PS: i dont think Shaun is your PC from either FO3 or FNV, nor is it James, seeing obviously they are named differently. You can name your PC's so unlikely they would now retcon it to be Shaun regardless of choices made.)

You were force to be a lobotomite before if you play OWB, although lobotomite is not synthetic human, it is not normal human either

Since I cannot play as non-human like ghoul/super mutant/intelligent deathclaw, I'm really looking forward that he/she is synthetic human.

I believe that Vault 111 is a CryoLab facility.

Right now I'm going with firing range instructor. It's how she met her husband.

you forgot buick rogers i

because this whole idea of this protagonist is a almost complete cpoy paste / hommage to this early 20 th century comics hero

in the original comic books he is a WW I veteran , and something happens and wakes up in the 25 th century , earth is a wasteland except for a few high tech cities who are almost constantly at war with each other and where his knowledge about warfare is more valuable than any technology

Well all we 'know' so far is that the protagonist is a 200 year old veteran (probably army if the leaked casting script is correct).

I think the belief that just about everybody could be an android is being over played, Harkness is important enough to have an Institute member with a human looking android bodyguard chase him to DC so I doubt human looking androids are the norm, it seems that most synths look more like early terminators with plastic skin. Also why would you implant the memory of a random 200 year old retired veteran into a top of the line android anyway?

Since he/she is in a vault, he/she is automatically a probable test subject for Vault Tec.

The alien abuduction is seems unlikely since the Fallout aliens seem to keep their specimens rather than returning them.

I highly doubt he is the father (or she the mother) of any of the earlier protagonists, possible ancestor maybe be he/she could also potentially be almost anybodies ancestor in any of the Fallout games......also unless its someone like the BOS or the Enclave, it unlikely that the average wastelander would even know about a relative of their direct ancestor from 200 years previously. A ghoul might know (or be related to) the protagonist from the pre-war era and that is one of the theories about the fate of the family.