I know Marcone and Thomas are popular candidates, but I just do not see either as possible.
Thomas is a full vampire. He feeds on souls - no choice about it. A Knight would not be allowed to feed on souls. If Thomas managed to kill his demon/vampire part, then a Knight could be possible.
Marcone is a strong individual with a strongly defined "bright lines" in his morality. But he is NOT a moral individual. He can certainly be a heroic figure in the dresden universe if the circumstances are right. He can (as stated) make an outstanding King or ruler or even Ally. He has a tiger's soul - and it would take something very dramatic to change that. And really seeing nothing, not even the recovery of the girl in the coma, changing that.
Fundamentally, for me at least, the morality of the Knights is the essential requirement. It is this combination of self-sacrifice and desire to do good that makes a Knight. Thomas does not have that for anyone beyond his immediate family. Marcone does not have that period. A person can go through a transformation moment (like Sanya) that results in them rising to that Standard, but it must be profound to have that effect. But I have not seen anything in the series to date that implies such a transformation for either Marcone or Thomas.