Think About This!!!!
That's not unusual for a weekly manga not to plan the rough plot more than a few months in advance.
There had never been the slightest hint of shodai or any other hokage being part of a clan until very recently. In the asuma arc the sarutobi clan was brought up out of the blue, and now it's the senju clan. Considering all the hokages have just been hinted at being senju, Kishimoto might even have forgotten about his other recent addition (sarutobi clan).
These clans, the rinnegan, Naruto's mom, the yondaime-like frogs and their prophecies, the elemental affinities/chart...these are all brand new additions that Kishimoto undoubtly had no idea he would ever introduce a year ago IMO.
Wether you like these developments is up to you but they have certainly not been planned all along.