#2 Adding new members in nWo
The star that would help Cena in his championship match should be established names, giving more credibility to the new nWo. Finn Balor and Shinsuke Nakamura are the best options since Balor has his Bullet Club roots and Nakamura will bring in an international feel to the stable. He'll certainly bring in more viewers from the Japanese market, just like Masahiro Chono did when he was with nWo.
The three of them should be the top heels of their respective brands, making nWo a part of the three brands.
WWE could then slowly add more members, with the likes of Elias, Sheamus, Shayna Baszler, Dolph Ziggler, Bobby Roode, Cesaro, Bayley, Damian Priest, El Legado del Fantasma, and Shotzi Blackheart amongst the possible names. WWE could even bring back a veteran like The Big Show who could act as the faction's enforcer.
Since John Cena won't be around much with his limited schedule, adding another frontman to nWo would be the best move, and AJ Styles is the finest replacement for Cena. The former leader of Bullet Club has risen up to become one of the greatest wrestlers of all time with his incredible skills in the ring and on the mic.
Having representatives on each brand will make nWo more dominant and stronger. They can even establish a sub-group on NXT UK and naming it as nWo UK, so they have a wide reach. The group as a whole can invade each brand within the company, thus causing chaos in WWE.