All of these Ryuki references... has me thinking I should watch Ryuki.

They won't kill off the major four, not until later in the series.

Gaim is the focal point of the story - he has to live for as long as possible.
Baron is the main nemesis to the hero (Gaim), but with flashes of being a hero himself, meaning he'll eventually fight for good in time, and thus cannot die until after that happens.
Ryugen and Zangetsu have to play off each other to develop a story and conclude a story before either can die. I, like others, believe that somehow Ryugen will succumb to darkness and become a villain while Zangetsu becomes a hero. Role reversal - the younger sibling rebels so much against what the older sibling wants that he becomes exactly what he didn't want to become, while the older brother attempts to save him.

My guess is actually Kurokage. The producer, including Urobutchi have constantly fawned over Gridon, saying that he's an interesting character. Why not Kurokage? I'm guessing Kurokage gets killed (perhaps by Bravo?) and Gridon goes through a shift in personality as his best friend dies.

Something needs to trigger Gridon into the "secondary Rider" status the show seems to have given him.

I could actually see this happening.

Except Ryuden is the secondary rider.

As long as they release a toy or two of Kurokage, then I'm happy.

How do you know they were humans? And people being turned into monsters is nothing. When was the last time an actual rider died?

As long as he gets the AC and S.H. treatment I'm happy.

Oh, what I mean is that all four Gaim, Baron, Ryugen and Zangetsu are shown to be the main ones that appear in the opening credits. So why on earth was Gridon in the press conference alongside them? The only ones outside the four main Riders in the press conference were Mai and Gridon, and as we know, Mai is a main character. Also, why was he in the Rider War in the beginning? Besides that, Urobuchi has said Gridon is his favourite character and to expect something from him in the future.

So I'm guessing Gridon will undergo a change to become a more significant character and maybe Kurokage's death would be that catalyst.

I think Bravo will die sometime soon even if it's by Zangetsu because he doesn't want him to be a Rider, I also don't see his potential rather than being a pain in the ass for the others, unless he becomes a powerful ally. I think Kurokage and Ryugen will die too later in the series, I agree something might need to trigger Gridon's evolution into a stronger character before he ends up being Punch Hopper. Ryugen I see dying in the sense of triggering something to affect both Gaim and Zangetsu or that he succumbs to the darkness from both how he acts with Cid, Mai not giving him the attention he wants, and not being able to live the life he wants and goes off the deep end leading Zangetsu and Gaim to try and bring him back. Basically I'm agreeing with the theories I read above. I am very intrigued to see where this series will go.