Odium possessed Elhokar... no thanks. If Elhokar goes evil/becomes an antagonist, it should be because of Dalinars mistakes, and his general situation, not because of random possession. That has rarely been an interesting storyline. The most interesting villains are the ones you feel symphaty for, and the ones who acts for understandable reasons. Elhokar is a character who could be such a character. Making him Odium-possessed would be a waste of time.
Furthermore, I disagree with @maxal. Elhokars "becoming a better king" arc wouldn't be the same as Renarins arc. Especially if Elhokar doesn't become a Radiant. Learning to be king, confronting his mistakes, and finally overcoming the fact that he is the average joe, would be a good arc for Elhokar. He could be in charge of holding stuff together at home while the Radiants are out killing things. And this would allow Elhokar to show character growth by seeing everyone but him becoming KR, but deciding not to whine and instead focus on improving and cultivating the skills he already has. Having Elhokar finally deciding that he can be himself, and be good because of it, instead of wishing that he was someone else would be a good arc.