Willpower is a no brainer imho.
Ivankov seeing Dragon in Luffy is also self-explanatory, their sense of justice is similar. What bothers me the most, tho not like most needed, Garp never referred to Dragon as his son if I recall.
He always says "Luffy is a son of Dragon the Revolutionary". He never says something such as: "My son is the head of the Revolutionary Army" blah blah.
We will see. Remember that we might both be wrong, Oda will retcon everything and Crocodile will be Luffy's mother