There is only one roll I can see that Arnold might fill and actually add to the film. I think having him cast as the Grandfather would be hack. But I think that the roll of Crom could work very well. Maybe a dream sequence of some sort.
Conan's army faces overwhelming odds against far superior numbers from a land accross the sea. He tries to hold the battle until reserves from Aquilonia come to aid, but the enemy attacks and the battle insues. Days later the Aquiloinan reserves arrive at the battle field only to find thousands dead, and one living witness to tell them the tale of the great battle. Conan was the last man standing and slew the last of the invaders himself. Sufferering from severe wounds he mounts a horse and rides slowly off not to be seen by man or woman again. Decades later, men of Cimmeria begin to spread throughout Hyboria telling tales of a god. Not the old cruel god Crom, but thier new God...Conan, And the tale of how he slew old Crom and took his rightful place as the new God of Cimmerians....or maybe he died of his wounds in a ***** house in the next town. Who knows.