I like Matrixbeast's idea. For power-ups/new forms, the viewers who support the protag or other Riders can 'donate'/'suport' them, giving them access to a new arsenal or form. Similar to how on Twitch you can donate money to support your favourite livestreamer or like Hunger Games where they can send a contestant something to help them in the contest.

EDIT: Easier weapons or forms could be found in the game but possibly, stronger forms/weapons need a certain amount of support from viewers to access it. Thus, this could make the game more interactive for them as they can help determine which person comes out on top.

Kamen Rider Hunger Games. :lolol

Hahaha. "May the odds and your status as the main protagonist be in your favor"

Maybe something more like an alien invasion has occurred in the past for the city. A giant corporation has teamed with the invaders, and the city now works more like a post apoc or fantasy setting as in its looser law, small factions and territories. Just spit balling here