I don't think anyone's mentioned this, but I feel Radiant Dawn's story would be a lot better if Sephiran, who's supposed to be the main villain, actually showed up sometime before the second-to-last chapter. I guess the reveal that Sephiran had been your enemy the whole time was supposed to be a big betrayal thing, but it's hard to feel betrayed by someone I barely remembered from the last game. If he had spent the rest of the plot pretending to help you only to reveal he was behind all your problems, not only would he be more interesting, but the plot would feel much more cohesive, rather than three stories that have little to do with each other.
Admittedly, I've only played RD once a few months ago, so my knowledge of the plot might be a bit flimsy.