This was touched on in other threads, but I will repeat it here.
The feelings and affection that bounce from a Rider to their Dragon are not direct feelings. In other words, since Eragon fell in love with Arya, Saphira won't fall in love with Arya. She will feel compassion towards her and a level of respect, but no romantic feelings. Same goes for the reverse.
Now, if we had two Riders and their Dragons, and the Riders fell for each other, the mutual affection that is shared by the Dragons would influence them to fall for each other. Hence the theory that Arya will be the next Rider and influence Greeni to mate with Saphira, but that is a topic for other threads.
But if Thorn and Saphira fall in love, that could help Murtagh and Nasuada come together. Same for Eragon and Arya. Then if in any case Saphira and Thorn have a daughter (wich may turn out to be violet), she could mate with the unhatched green dragon.
1. I don't see how Thorn + Saphira will help Murtagh + Nasuada. I don't have a problem with either relationship, but they aren't realy connected.
2. You know, it's not like the green dragon is a baby and Thorn/Saphira are adults. If it hatches in Book IV, it would only be a year behind Saphira and Thorn. Unless Thorn and Saphira mate extremely early on, I'm thinking their offspring would be quite a bit younger than the green dragon.
btw, is it stated that a dragon's color is determined by the parents'?
I don't know about the color.
But I think it's a superb idea if Thorn and Saphira mated and then the green dragon will have feelings for their daughter.
What I meant was, that both pairings are very similar. It make sense that Murtagh may have feelings for Nasuada.