Actually, Tarantino has been asked that question a gang of times and said "I'll leave it up to the audience to decide whats in the briefcase". So, 2 things have become dominant.
Some say in the briefcase was Marcellis' soul, they were trying to return it to him, thats what the bandaid indicated(where the soul was taken out). Also, some say it was the "diamonds" from Reservoir Dogs. Another theme in the movie is Samuel L Jackson's role of being changed through events in the movie, he kept repeating a verse from Ezekiel 25:17
The path of the righteous man is beset on all sides by the inequalities of the selfish and the tyranny of evil men. Blessed is he who, in the name of charity and good will, shepards the week through the valley of darkness, for he is truly his brother's keeper and and the finder of lost children. And I will strike down upon thee with great vengeance and furios anger those who attempt to poison and destroy my brothers. And you will know my name is the lord when I lay my vengeance upon you.'
Well, his role defines the verse throughout the movie. peace