How long have you been reading OP? I've read it in a span of a week(1-582 at the time) so, I couldn't have made predictions regarding those 2 and at that time I wasn't interested in who Dragon was and I treated the coverstories at that time, something that the author would do just for the heck of it. Pretty good job.
I guess the online community had already struck Water 7 when I started, but personally I only read the Swedish Volumes in the order they came out at the time. Me and my brother discussed it every now and then, and eventually we hit the topic of Luffy's parents. I wondered if the father character may not have already appeared and Dragon caught my eye. Volume 16 revealed him with black hair and I figured, maybe that's the one.
As for Garp, I was thinking about how Buggy had reappeared in the company of the shady girl revealed to be Arvida. I figured Coby's story might tie in similarily and Garp seemed to have a bit of design to him. Him being Luffy's grandfather on the contrary, I did not see coming.