Most people will immediately think that nasauda will become the new leader however I disagree I think it is more likely that Roran will be leader. I don't think that Roran will strive to be the leader but nasauda will die and he will be offered the post an accept.
Reasons why:
1. Nasauda dying will continue Murtagh's bad luck considering that it seemed that they liked each other. As a side-note I also think that Thorn will die.
2. Nasauda has a hatred of magic and it's users which will cause conflict if she rules.
3. Elva might resent Nasauda for trying to keep her shackled to pain and assassinate Nasauda.
4. Nasauda may get killed by Galby during the attack on Uru'baen.
5. Roran is a good leader and his men seem to like him and are willing to follow him anywhere.
6. He is willing to suffer for his people.
7. He is a good orator.
8. He is a friend of eragon and all the leaders seem to be getting replaced with Eragon's friends. ex. Orik replacing Hrothgar, Nasauda replacing Aijhad, and Arya poised to replace the elven queen.
9. It seems to me that throughout books 2 and 3 he has been groomed to take up the post.

I suppose you're right. I just think Nasuada is a better orator.

Those are really good points, AC. I agree. Roran seems more likely to be the next ruler. He has shown that he can lead and call the shots without cracking under the pressure. In my oppinion, he would make a very good king and be fair and just to his subjects.

you all seem to be forgetting something, roran made a wedding vow (which in the books is the highest vow you can give, besides in the AL) that he would go back to his farm and rebuild it and forget about everything that happened. he cannot become king.

Actually Horst says on behalf of Roran that Roran will brings the promise, of a farm in carvahall where they both may live in peace, to the marriage. He could still have a farm in carvahall and be king after all the first human kings in Alagaesia ruled from Palcanar valley.

I think if Murtagh survives he and Nasuada will get together and rule alagasia.

You seem to be forgetting that there are descendents of royalty in Carvahall valley.... i forgot who said that tho but Roran wil rule for he will be discovered to have royalty in his blood

What would Nasuada do then.

Murtagh will never rule. He is a Dragon Rider and will live a long time or forever. No one would support a ruler like that after Galby. Roran might not also. Everyone in Palancar Valley are descendents of royalty so it could be anyone from there if it was one of them. Like others have said Rorn doesn't want to lead. Concerning Murtagh and Nasuada, I don't even see how people could see them being together. One will live a long time and the other won't. Also, Nasuada only visited Murtagh once or a few times so it's not like they saw each other a lot.

Murtagh really liked her. If you read the part where Eragon goes to visit Murtagh in his cell you'll see that.

But they never spent any time together hardly at all and never said how much they liked each other. Plus, he will live on centuries after her and he knows that. He wouldn't be with countless people.

I hate the idea, but maybe Nasuada dies.

I don't think it will be Nasuada. Ajihad already died.

I originally took that point of view but I can't help thinking that despite the fact it looks like Roran would be a great king he wants to settle down and become a farmer and you can't force people to become leaders.

A possible king could be Horst as he was the primary authority in Carvahall for many years, or jeod as he is already familiar with political systems and is educated. Or the leader of the nomad tribes who challenged Nasauda to the trail on the long knives or the captain who met Saphira, Eragon and Orik in Aroughs. The list goes on and on. However at the moment Roran's primary goal is to defeat the empire so that he can settle down as farmer with Katrina.

i think roran is a royalty, descendant of the long line of the king palancar. but i dont think he will rule, he made it clear in many ways that he just want to go back home and do farming with katrina. may be he will become someone important in the new establishment.

i am pretty sure that it will be nasuada who will become queen, remember in book III elva said to her something about queen, i am not sure what i have to look it up in book.