Protag Ryoma huh, I don't think he suit to be protag of this wacky story, but, here's some of my thought. or maybe more of ranting.
So, my first thought of this would be if both Kaede and Shuichi got executed. (maybe they got the same votes somehow)
Ryoma, feel responsible for it (like how he offer his life but no one take it but he still feel bad about it.)
And with both Kaede and Shuichi being executed, he could feel that something isn't right. make him have some motivation to do during this killing game, which is find the truth of why this even happened in the first place.
during chapter 2, video tape being given to each student. surely, Ryoma would want to watch it, same go with Kokichi. even other students all agree not to watch it. It was enough for Kokichi to spark interest in Ryoma more than others.
Kokichi would follow Ryoma around, sometimes dragging Gonta along. from other students perspective view, they almost look like a small criminal group.
Kokichi would soon became Ryoma's investigation partner. as in canon, Kokichi was one of the people that admire Ryoma pretty much. mostly in gambling but yeah.
As they spend more time together, Ryoma slowly become trustworthy for Kokichi, make Kokichi trying to keep Ryoma alive in his own little liar way.
would he batray Ryoma in the end? who know
talking about partner, Kaito is the next subject to talk about. in canon story, Kaito despise Ryoma for giving up on life. so it would make sense that he would become Ryoma's antagonist.
yes, I love Kaito. but his favoritism and bias is pretty strong too. but I love that as character's flaw.
Then, as chapters go, Kaito could have character development, learning reason why Ryoma, the one he used to admire so much, become so depressed in the first place.
and in the end, might be the person who's protective over him the most.
That's all idea I have right now.