Other than Mamoru, I think Nephrite, Saphir (after finding out about the Death Phantom in the 90’s version), Asanuma , & Motoki would make great candidates.
Nephrite- He turned good, and seemed like he would help the Senshi had he lived. Naru might have been able to become a Senshi, or at the very least more involved with the plot and have more screen time in later seasons with Nephrite aiding the Inner Senshi.
Saphir- Same reasons as Nephrite as he started to see Earth and earthlings in a new light or perspective. With him also being involved, the Ayakashi sisters would also be more involved or get screen time (at least Petz would).
Asanuma- Since he’s Mamoru’s friend, it’d be easy for Mamoru And him to get along
Motoki- Same reason as Asanuma, though I’m not sure how he’d take the whole superhero and magical aspect of it all.
Nephrite- He turned good, and seemed like he would help the Senshi had he lived. Naru might have been able to become a Senshi, or at the very least more involved with the plot and have more screen time in later seasons with Nephrite aiding the Inner Senshi.
Saphir- Same reasons as Nephrite as he started to see Earth and earthlings in a new light or perspective. With him also being involved, the Ayakashi sisters would also be more involved or get screen time (at least Petz would).
Asanuma- Since he’s Mamoru’s friend, it’d be easy for Mamoru And him to get along
Motoki- Same reason as Asanuma, though I’m not sure how he’d take the whole superhero and magical aspect of it all.