I believe that the title of the finale book could be Eldunari. This could be possible due to the fact that Eragon is now going to be searching for a way to separate Galbatorix from his store house of Eldunari. Also I see the Eldunari having a very big role in the final book. Another good title would be Unbitr. Unbitr would be a great title for the book because it goes with the whole Eragon Inheritance thing Paolini is doing. It would be cool if Eragon didn't come across his father's sword until the end of the book and then took it for his own, thus inheriting his father weapon.
Eragon's True Name
Ever since the sword Brisingr was introduced I've been wondering about why the sword acts the way it does. And I think I've come up with the answer. Eragon made Brisingr, he put his heart and soul into making the weapon. What if he poured a piece of himself into it, thus naming it Brisingr we now have part of his true name. There is proof to this theory. Remember Eragon had asked Oromis the day before if he could discern his true name, and Oromis replied he probably could but wouldn't. Then when Oromis learned of what was happening when Eragon said Brisingr when holding the sword he took on a thoughtful expression saying "I wonder?" Oromis more then likely put two and two together and came up with this.
Now knowing this it is I came up with a possible True Name for Eragon. I think it would be cool if his true name were something like Fire of Change, or the Fire of Hope. Now both of these would be good because both are saying that Eragon is the one, the only one who can defeat Galbatorix. That Eragon is the light in the darkness that is the Empire. It's only a guess but I can easily see this happening.
Rock of Kuthian/ Vault of Souls
This one I've been giving a lot of thought to lately and I think that the Vault of Souls is basically a old, forgotten storehouse of Eldunari that was used before the time of the Dragon Riders. Also I believe that Eragon I and his dragon are keeping watch over the vault. This being said for Eragon to open the vault he needs to open the vault, and the only way to open the vault would be to speak Eragon I true name. Now characters in the book have mentioned that it is strange that a farmer would have the name of the first Dragon Rider, it's also strange that both Eragon's have been the first of a new Order. It has also been stated that the name Eragon holds power and its rarely ever given. This to me show's that there is a possibility that the name Eragon has some sort of power over a certain object, perhaps the vault.
Eragon I was the first Dragon Rider, and Eragon Bomsson first of the new Order of Dragon Rider's. That got me thinking, if they're so much alike wouldn't it be cool if both of their true name's were the same. As this power that Eragon is to receive at the Vault would be the Eldunari of Bid'Duam and the other Eldunari within the vault. What would be more powerful than all of the oldest Dragon's in Alagesia. Also I can see the Dragon's willingly giving Eragon their power instead of him taking, not that I can see him doing that. This way he doesn't have to make them summit and I can see them passing on their wisdom to him as well as their power.
Also I have a theory on the vault itself. If the vault is a way storehouse of Eldunari then perhaps the dragons of old in a deep emotional state to protect those Eldunari used magic so that everyone would forget about the rock. This is more than likely the most believable reason that no one can remember anything about it but always say the name sounds familiar. The dragons have done this before, they did this when the Forswon and their dragons sided with Galbatorix, making everyone forget about the dragons, and causing the dragons to lose their intelligance and regress to beasts. I'd also like to think that The Rock of Kuthian was located in the Hadarac Desert, Saphira seemed to feel a need, a want, even a primal urge to want to stay there. This is most likely due to the dragons enjoying area's so hot and humid. This would be a perfect location that for Dragons to store their hearts. It's a place where no one would want to go or could survive.
Murtagh and Thorn
Murtagh and Thorn's fate is a question that everyone wants to know. Will they die, free themselves and turn to the Varden, what? I personally would like to see them turn to the Varden after freeing themselves. To me the way in which they change their true name is liberating the green dragon egg from Galbatorix and as many Eldunari as they can take. Even though Eragon doesn't like to recognize Murtagh as his brother he doesn in some small way care for him. This was shown in Eragon's pleading with him to turn sides, and his wanting to capture rather than kill him. Nasuada seems to have some kind of feelings for Murtagh, and I doubt that she would want to kill him. Him bringing the egg and Eldunari would show the Varden that he's changed and wish's to join them. The Dwarve's however wouldn't be as gracious to accept him after killing Horthgar, it's been said that they hold a grudge like no one else. Even if Orik decides to accept him he'd have to do what his people wanted or he'd have a clan war on his hands. The elve's as may accept him at first due to how they view Dragon's and riders, though they may show some hostility.
Another possibility is he dies. I'd like to think if this happens it's because he died from injuries received from Galbatorix for betraying him. Paolini could have Murtagh steal the egg, him fight with Galbatorix which he becomes gravely injuried beyond repair, and as a last act takes the egg to Eragon.
Eragon's True Love
Arya is the only one that I can see as a fit for Eragon. He has loved her from the moment he saw her, and while she has been cold towards him at times you can see that she does care for him. In the first two books her feelings were always hidden, though would every once in while show themselves. In my view she is developing feelings for him. When Eragon showed her the fairth he made of her, she destroyed it and ran. I doubt that she did this because she dislikes him, but because she didn't want to be a distraction for him. In the Brisingr we see her opening up more to him. She seems to only ever open up to Eragon. Arya tells Eragon that before Faolin died she was different, she was more open, more cheerful, like the elves that Eragon has met. Arya healed a injury that Eragon had received that he could have easily healed himself. Finally when he was about to face Murtagh she told him "Be careful, Eragon. I would not want to see you broken by Murtagh. I..." I think she was about to tell him that she loves him, but then second guessed herself. In believe that Arya's thinks that everyone she cares for dies, her father died and Faolin died, so if she doesn't voice her feeling's Eragon wont die. I think she'll overcome this fear in the finale book and voice her true feelings.
Nasuada I think isn't a choice at all. Her and Eragon do feel something for each other, but only as brother and sister. It would be too weird for them to be together.
Tenga and Angela
I think both Tenga and Angela are either descendant's of the Grey Folk a Grey Folk themselves. It's really the only thing that make's since. Oromis basically told Eragon that Angela is older than she appears. Angela also somehow always seems to know when something's gonna happen, this could be a special power or ability that the Grey Folk have. Tenga we don't know much about. He's a whole question ark on the series other than what Angela said about his character. But we know that he wise, wiser than we may know. It's possible that he is trying to re-answer things that the Grey Folk had learned. It's also possible that the answer that he is searching for is how to use the energy of light. If so this will give Eragon an advantage over Galbatorix.
Angela told Eragon that he's to live for a long time and leave Alagesia never to return. The part about him living for a long time means that he'll defeat Galbatorix. The part of him leaving Alagesia is trickier to know. My guess is that once Galbatorix is defeated they'll rename the land. Remember Alagesia wasn't always it's name, it used to be called the Brodding Kingdom until Galbatorix renamed it. I think it would be cool it were renamed the Brodding Kingdom. Therefore Eragon leave's Alagesia forever.