You might be right on Mashimo/Miyahara specifically, but I look at how the junior tournament was booked and see no reason why this one would be any different.
Junior and heavy divisions are night and day tho. The junior division is a skeleton crew, Aoki, Hikaru, Sushi and Yohei. The heavyweight division is bigger with guys like Kento, Jun, Omori, Jake, Yuma? Zeus, Bodyguard and the regular freelancers like Irie, Hoshitango, Shuji etc.
My personal take is that guys coming in get more wins to help establish them, and with time they'll fall into the pecking order. Just lookign at the super power series poster it seems like Kengo, Sai, Daichi and Strong BJ will be at least semi regulars going forward. Granted it also comes with the territory of AJPW establishing itself on this current level they are in.