Ok i dont know if anyone's ever posted a thread with the same theory but here it goes =].
I think Shanks is Ace's dad because:
1) Luffy considers Ace to be a or like a brother to him. Could that be because while Shanks was still in town he brought along his son with him and they got to know one another? We're also pretty sure Ace isnt Luffy biological brother. But what we do know is that Ace left Luffy's hometown 3 years before he did. Mayb Shanks left him there because he didnt want Aceto have the pirates life. Perhaps thats why he joined up with Whitebeard instead of Shank's crew.
2) Shank's complete name has been kept from us since the start. And we all know that all the big condenders for One Piece has "the will of D". Monkey D. Luffy, Marshall D. Teach, Portgas D. Ace, Monkey D. Dragon, [theres alot more but we'll stick with what we got]. I just think its more than a lil likely that Shanks has a D. somewhere in his name.
3) When Shanks heard about Ace fighting Blackbeard, he pleaded with Whitebeard to call him back. He even went so far as to clash sword with polearm with Whitebeard.
4) Ace does look alot like Shanks [cept with the absence of red hair and having freckles]
5) Portgas D. Shanks has a nice ring to it xP [weak point haha]
Like to know what everyone else thinks!