Christopher Paolini has often been asked this in interviews.
His Word of God answer is usually No Comment.
Going back to when Eragon and Saphira coaxed the Menoa tree into giving them the Brightsteel – does the momentary pain in Eragon’s stomach have anything (or everything) to do with what the Menoa tree wanted from them?
Whatever the Menoa tree did or didn’t do, her plans are as long, intricate, and slow-moving as her consciousness.
What did the Menoa Tree take from Eragon?
The Menoa tree did take something from Eragon, but I can’t say what without spoiling my next book set within Alagaësia.
Please, please, please, share your knowledge... What did the Menoa Tree take from Eragon?! Or is that a "No comment" question?
No comment. :-)
But he does say that we will eventually find out in the later books, specifically book V.
Will we ever find out affirmatively what the Elf tree took from Eargon?
Before Inheritance came out, you always answered spoiler questions like "Go read Book Four" for such questions like "What did the Meona Tree take?" It was never revealed what the tree took in that book. When you answer such questions with "Book Five" do you really mean that these questions will be answered in Book Five?
Yes. Those questions will be answered in Book V. (smart of you to ask!)
Paolini also has confirmed that "Go" meant "leave Algaisia".
Fix me if i'm wrong-the Menoa Tree told Eragon to go..so she had probably meant that she wants him to leave Alagaesia, right?
That's exactly right.
Whatever it was that she took, it wasn't his or Saphira's ability to have children.
Can Eragon have children? Or did the Menoa tree take that?
Eragon can still have children (though given the changes from the Blood-Oath Celebration, might be difficult w/human or elf)
Can Saphira still have children?
Of course. Dragon eggs for all.
It also wasn't his moral regard for life.
I think I've figured out what the Menoa Tree took from Eragon. -a small part of his moral in regarding life (or maybe Eragon's regard for life is just disappearing slowly in general.)
Alas, no -- but it would certainly make for a great metaphor! You should be proud of your idea; it's one of the best I've heard so far.
It wasn't even his appendix.
I vaguely remember that it was a twinge in his lower belly that happened when the Menoa Tree took something from him. Nowadays I'd have guessed it was his ability to have kids, but as a kid reading the book, I had just learned about how useless your appendix is, so I kind of assumed it was his appendix. Maybe useless to him, and food for it, or maybe has some other use that he'll have to compensate for later. Are either of those correct?
Ahaha. No comment. (It's not his appendix, though.)
Paolini says the answer is possible to find out by reading the books, but he hasn't actually seen anyone fully get it yet.
Has any fan guessed correctly what the Meona Tree took from Eragon? If not, is it just not possible to guess based on the books?
Someone may have correctly guessed what the Menoa Tree took (I haven't read every theory out there), but as of yet, I haven't seen a correct guess myself. Some readers have gotten really close to the truth, though. It's certainly possible to guess based off the books.