Why Jinchuuriki Obito Resembles Rikudou Sennin
Obito, upon finally taking control of the Juubi's Chakra, transformed into an appearance resembling Rikudou Sennin:
Obito's staff btw is not Rikudou's staff but a replica protruding from his black chakra:
Rikudou Sennin's staff is a real item.
Naruto, upon obtaining Kyuubi Chakra Mode, also began to resemble Rikudou Sennin:
Naruto, upon obtaining Bijuu Mode, changed the appearance of the seal on his belly:
The swirl design of Naruto's KCM Seal transformed into two concentric circles.
But why two concentric circles only? why not Four Concentric Circles like Obito's?
It is because Kurama is missing its half: its Yin-Chakra
It is my hypothesis that the reason Obito and Naruto resemble Rikudou Sennin is because their Jinchuuriki Seal have the same origin: Uzumaki Fuuinjutsu. And that Naruto's Seal will transform into something similar to Obito's when he receives Kurama's Yin Half.
Even Minato, upon accessing Kurama's Yin Chakra with his Shiki Fuujin Seal, also enters into a mode that resembles Rikudou:
That same seal appears on Minato's KCM Mode the same place where the Shiki Fuujin Seal was: His Chest:
The reason Obito, Naruto and Minato resemble Rikudou Sennin is because they are using Uzumaki Fuuinjutsu: Rikudou's own Fuuinjutsus.
The Viz translation says that the Jutsu of becoming a Jinchuuriki was created by Rikudou, Kushina said that the Jutsu itself (and not just the type but the very Fuuinjutsu itself) is of Uzumaki Origin:
And who was the second Jinchuuriki in history after Rikudou Sennin? Mito Uzumaki:
That is why there were no Jinchuurikis prior to the alliance of the Uzumaki Clan with Konoha, because the Uzumakis have the original Fuuinjutsu of becoming a Jinchuuriki. The Uzumaki Fuuinjutsus are Rikudou's Jutsus because Rikudou Sennin himself was an Uzumaki, that is why those who uses the Uzumaki Fuuinjutsus to
access and successfully controls the Bijuus/Juubi's Chakra resemble him. They are Rikudou's jutsus, Uzumaki Fuuinjutsus.