No jiriaya stated clearly in canon that minato created the jutsudidn't jiraiya teach it to Minato? I know it was the 4th Naruto shippuden movie and it's not really canon but still
Minato Namikaze created the Rasengan and He showed Jiraiya and Kakashi how to use it was an incomplete jutsu. Naruto learn it from Jiraiya and He showed Konaharu how to use it. Naruto was the first to add element to it and make it a complete jutsu. Not every ninja can add element to the Rasengan. And sines it a Namikaze/Uzumaki invention at this point and time they're not going to be many users. But ones Naruto and Hinata have children they're going to be run all over the village shouting out Rasengan thanks to the Uzumaki Clan.
Not Orochis style, It would fit Neji though since it's a spiraling ball of chakra, kinda like Rotation.You're saying rasengan has very few users? It went from just Minato and Jiraiya to Minato, Jiraiya, Naruto, Kakashi (showed it later), and Konohamoru.
While chidori has two users. Kakashi and Sasuke. That hasn't changed. I do think it would've been cool if Orochimaru or Neji was somehow able to do it.