As far as im concerned, with a new director in the 3rd series, i could see athrun and lacus comin back
There were already a few hints that lacus mite have a soft spot left for athrun and she probaly knows that athrun has a place in his heart to forgive himself and forgive her to give it another chance.
In destiny, when she met him after landin IJ, she asks him hows he doin, and she had a different look at him. Then mia died, lacus looks up at athrun sheddin a thousand tears for mia, and she also shed with him. Then she goes through mia diary, about her experience with athrun etc, maybe just maybe lacus could see the pain that she has cause athrun into all these relationships and these girls were like replicas of her.
And do you really think ppl on plant will allow lacus to marry kira? remember the public on plant doesnt know about re-arrange marriage is canceled between lacus+athrun, this will probaly tarnish sumones (lacus or athrun) image at plant. Lets hope the new director looks at pride/honor first for the zala family, and lacus kleine will have to be athruns wife!!