LOL at the biggest fights in history. I think all 3 of Brock's fights were, at the time, the biggest in history but since then, at least BJ-Penn has surpassed it.
I don't think the UFC would allow Fedor to come in for one fight. Although Lesnar is a challenge, Fedor has so many more ways to win. They wouldn't want him winning and then walking away.
The UFC is willing to give him lots of money, but unwilling to give him equal contractual terms. I think they are worried that they can not make one exception to their standard contract (automatic renewals, ability to drop fighters who lose)