As far as the main characters of GSD basically we have them following the 3 most basic paths of developement a character can have in a sequal.

Kira in seed was the ultimate quardinator really the end all of the series from start to finish. Yes he developed a lot in GS and eventually he settled on being who he is now. His character is there as basically a way to measure new characters strengths and to please the fans. In the end he isn't a big enough character to develope as his role is already to well established to change.

Athrun is the character who people were unhappy with at the end of Gundam Seed. In the end he really ended up the same as he started. In both GS and GSD he has almost EXACTLY the same developement. from start to finish so maybe this time he will end up figuring out who he is and stop the cycle he's in.

Shinn is the new guy who essentially has to improve during the series in order for there to be one. Without a new character there is no reason for a new series as we could just go rewatch seed and be a lot happier. The downside with Shinn is he's really the unwanted hero. We already have Kira as a main good guy. Also we already have Athrun as the main good guy who can't pick a side. So this leaves Shinn to be the bad guy who eventually finds redemption. Essentially he has Athrun's role from GS but on a scale that seems so unimportant and petty compaired to having your dad be the main figure for Zaft. Essentially Shinn is the bastard child of Fllay and Athrun. His point is to be hated and quite possibly to have to die to achieve redemption. So people will hate him until the end reguardless and if he doesn't die we just end up feeling cheated.