Important secondary characters you feel will be difficult to make into believable MCs?
I thought of a couple of series in which the supporting characters, like siblings, are given a lot of focus and even some hint of romance with another character. Or it could be that it gets brought up as an aside comment that they really do not like a certain character.
There is a book I read (which is the first in its series) in which I had a strong feeling that I knew who would be potential future MCs. The problem was that one of those characters had been portrayed as so irredeemable and terrible that I have trouble thinking of how the author could make that romance work without really humbling the character/making them go through a lot of grief and compromising the intelligence and self-respect of the other potential MC. Not to mention that the book ended with the character effectively having alienated themselves from their family because of their hateful behaviour. The reason I thought they could be a future MC is because there were some instances of hidden depths implied in the character's past which seemed to lead up to something that happened to make the character so terrrible now. Also, the author has taken a similar character type and made them an excellent MC before so I wondered if there would be a repeat. Maybe it'll be a surprise and this character will remain an antagonist whose actions cause their undoing.
I'll give a better known example with the unreleased {A Sinful Kind of Scot by Mia Vincy} which is to be about the character Lucy from {A Wicked Kind of Husband} but her character was not portrayed as likable (for lack of a better description). The epilogue implied something about the character and I expect there will more depth to her, possibly looking at events from a different perspective and even some potential psychological reasons for her behaviour (as the author has discussed those in other characters in her books). But I still find it difficult to see justification for all her brattiness and self-destructive behaviour and feel the romance would involve a much older and maturer MMC and probably some much-needed apologies and conversations with her older sister to make it believable.
I suppose what I meant is that there are some characters I do not think I would like to read a whole book about in their present state, unless there is a lot of character development. They are fine in small doses. Do you have any such characters in mind whose romance and happiness you would not want to root for? I would say that Colin from {Rescued from Ruin by Elisa Braden} would probably have been a response of mine if his book was not already out and I even see many reviewers say the same thing.