But still, I can't see much in terms of Kira's possible character development... At least not enough to make him the main protagonist of such show. Same for Athrun Zala. Unless they make the "unthinkable" - they start to kill off important characters. But really, give me an example or some idea about them that may not sound cheap.
Personally, I'd not risk focusing all over again on KiraxLacusxAthrun. I'd just incvent the new story with them in the background. No old Gundams. Every single Gundam was destroyed because of peace treaty. Maybe only Akatsuki hidden deep under Orb. And then come the new ones. Two are enough for the beginning
But it's just me.