The PS is already a mainstream party by virtue of its electoral weight.
The reason why the PS is more left wing in apparence (but not always in substance) compared to say the British Labour Party, have a lot to do with the fact that since the end of the war the left is strong in French politics. Don't forget that for a long time, the main left wing party in France was the communist party itself. This changed afterwards for both electoral reasons and because Trotskysts and hard left wannabies politicians like Jospin realised that they stand more of a chance of being elected and gain power by joing the PS. This and the need for accommodation with the Communists decisively shifted the PS to the left. The Programme Commun of the 1970s, partially implemented in 1981 was a very left wing document which Michael Foot would have been delighted to implement in Britain.
Ever since 1981 one can make the argument that the PS become more social democratic and less socialist. But the PS never really needed to change its orientation, since the opposition it faced was never smart and organised enough to shift the terms of the debate in its favour and towards the right.
The French right has never really managed to unite around either a leader or an ideological programme since De Gaulle left power in 1969. It become rather conservative and reluctant to do anything during the seventies, partly through fear of aleniating its base and partly through a lack of organisation. During the eighties and nineties it can be said with hindsight that Chirac did a lot of damage to the French right. That man was a snake whose only objective was to gain power and fame for himself and his protégés. The right did implement a privatisation programme between 1986 and 1988 but it was a small one and it served no end save for privatising for the sake of privatising. The French right never developped a coherent ideological alternative to what the PS what proposing, whether on economics, foreign policy, social security and education. It carried on the foreign policy of De Gaulle, and protected its minions like private schools, unaccountable conglomerates like Dassault and Lagardere and so on. The rise of Le Pen Front National also emasculated it, the RPR and such could have nipped Le Pen rise in the bud, had they answered to the questions he was raising, but scared of the left and of themselves they did not.
This is more or less still the case today to a degree. Sarkozy is a snake just like Chirac and to say that he adheres to a coherent philosophy like some of his supporters do is laughable. Through his heavy handed approach to politics he has only managed to reinforce his opponents. If he wins in 2012 it will be by default for lack of a better alternative.
In order to make the PS more like the Labour Party, the answer lies in making the French right much more potent and much more coherent both philosophically and organisationally. Jacques Chaban Delmas New Society ideas offers a possible start on the philosophical front. They were radical for their time, but in line with the "neither reaction nor revolution" philosophy which animated Gaullism. Get his programme implemented and the rug will be swept under the feet of the Left (they were very scared of his programme OTL), forcing them to rethink and reinvent themselves. Getting the French right to be more organised and to develop think tanks and such is the harder part. But it can happen if the right men and the right decisions are taken.
The New Society program should allow the right to remain in power until the mid eighties. Eventually the PS will take over and if the New Society program has been really successful, they won't dare challenging its legacy. The communists will be on the fringe and possibly not even allied to the PS anymore.