Youssoufou Bamba said Alassane Ouattara was prepared to work with Mr Gbagbo’s followers in the wake of November’s disputed election.
Mr Bamba was appointed by Mr Ouattara, whom the international community recognises as the winner of the vote.
Mr Gbagbo has resisted pressure to acknowledge defeat.
Â Mr Bamba said the only thing that could not be negotiated was the result of the election.
Gbagbo “has been defeated,” he said. “He has to admit that, he has to step down and… the rest is negotiated.”
“Mr Gbagbo is not alone,” he added. “He has followers, he has competent people in his party. Those people, we are prepared to work with them. In the framework of wide composite cabinet.”
Mr Bamba repeated opposition claims that Mr Gbagbo’s supporters had carried out “a massive violation of human rights”.
“But you know in politics, life goes on,” he said. “You have to, at some point, envisage how you are, because you are condemned to live together.”
The UN says some 200 people have been killed or have disappeared in the past month – mostly supporters of Mr Ouattara.
It says security forces loyal to Mr Gbagbo have twice blocked peacekeepers from visiting the site of an alleged mass grave.
The Gbagbo government has repeatedly denied the existence of any mass graves.
A certain source says the idea of a unity cabinet had already been hinted at when Mr Ouattara named a small cabinet in December, filling only the essential posts and leaving others vacant.
He says that even before the election, Mr Ouattara said his cabinet would be open to all talents – an admission of the reality that there are a large number of Ivorians who find the idea of him as president hard to swallow.
Mr Ouattara remains behind a blockade at a hotel in the main city Abidjan, protected by UN peacekeepers and New Forces former rebels who control the north of the country.
November’s election was intended to reunify the country, which has been divided since a 2002 conflict.
Mr Ouattara was initially proclaimed the winner by the country’s election commission – a verdict backed by the UN, which helped organise the poll.
But the country’s Constitutional Council, headed by an ally of Mr Gbagbo, later ruled that he had won, citing voting irregularities in the north.