When men began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of men were beautiful, and they married any of them they chose. 3 Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years."
4 The Nephilim were on the earth in those days-and also afterward-when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.
5 The LORD saw how great man's wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time. 6 The LORD was grieved that he had made man on the earth, and his heart was filled with pain. 7 So the LORD said, "I will wipe mankind, whom I have created, from the face of the earth-men and animals, and creatures that move along the ground, and birds of the air-for I am grieved that I have made them." 8 But Noah found favor in the eyes of the LORD
Genesis 6:1-8
The Bible contains some pretty neat stuff about the life before the flood. According to Genesis, before the flood men were able to live several centuries (in verse 3, God decrees that from then on man's lifespan would be a maximum of 120 years.) Also in verse 4, Genesis talks about the Nephilim (I think that it is Hebrew for "fallen one") superhumans born of the union of mortals and fiends. The King James Version uses the word Giants for Nephilim. Perhaps, Gilgamesh and other legendary heroes were really Nephilim. Also there is also only one language, because it is before the tower of Babil.
An antediluvian campaign could be really neat. It definately would be very epic. However, since it would involve Judeo-Christian religion in it, it could potentially be pretty controversial, so it would be best to make sure that players wouldn't be offended.