[KRWolfblade]

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.