First of all let me point out the fact that Sanji being a prince has been forshadowed a lot, so when someone gets it right, it is not that he/she predicted it but rather Oda has thrown hints of it out there.
2nd I have always kept saying that Sanji will get a backstory because of the "I will never hit a woman". In Dressrosa Oda gave mr pink and Baby 5 a serious sad backstory to why they dress and say what they say, there and then it was a guarantee for me that Sanji is definitely getting one as well.
3rd- just like most of you, Sanji being a prince or world noble would be kinda not all that interesting since it has been done with Sabo and Doflamingo(except in Sanji case he would be the first world noble pirate, since Doffy wasn't one anymore).
But you know what hasn't been done yet? Sanji being the son or grandson of a Gorosie, the blonde Gorosie looks the youngest and is the only one who's hair is not white so it could be Sanji is related to him? Now that will be an interesting story to hear.
Last- I hope after the Sanji story that Oda puts Sanji back to his east blue self… Back then Sanji was a gentleman, funny and strong character.. Back then Sanji was going after women as a gentleman example: serving Nami food, having her not pay and calmly giving her flower nothing too extensive but now he goes after women all jumpy and shit, love eyes everywhere, love heart, turning to stone, bleeding, can't even talk properly.
Eastblue Sanji- funny gentleman who is strong and badass
Current Sanji- non-funny pervert, desperate guy who is just strong enough to be used as a hype in a badass way.
But guys you know the funny thing? Even after all this shit Sanji character has been through and being absent for a year he still came in as the 4th most popular character. Now imagine if Oda has been consistent with Sanji character till now, he will probably give Luffy a run for the 1st place.