Tongues are also likely to wag at the spring-summer shows over where John Galliano might go, with his contract at Maison Margiela nearing its end.
The first shows from the big-hitter French houses will come Tuesday with Dior and Saint Laurent, with a packed calendar confirming Paris's crushing dominance over rivals Milan, New York and London, reports AFP.
And there is no let-up at the end: Chanel opens the final day on October 1 by returning to the vast Grand Palais, the scene of some of Lagerfeld's most jaw-dropping shows, after an absence of four years.
The house is shelling out 30 million euros ($33 million) to stage its shows at the iconic Belle Epoque edifice, which reopened after a major facelift to host fencing and taekwondo at the Paris Olympics and Paralympic Games.
With Viard -- long Lagerfeld's right-hand woman -- gone, observers expect a collection drawn from Chanel classics.
In contrast, there could well be fireworks from Alessandro Michele, the mercurial Italian designer who transformed Gucci, who may be keen to make his mark with his debut show for Valentino.
Equally anticipated is French duo Coperni, who are staging their show at Disneyland Paris on the final night, with an after party in the theme park that promises to go on into the wee hours.
The brand's founders, Arnaud Vaillant and Sebastien Meyer, pulled off a coup with their outfit for Belgian singer Angele for the Olympics closing ceremony, and are clearly in a mood to celebrate.
Another hot duo, the Olsen twins, the Los Angeles child actors turned designers, have kept their place for their luxury line The Row in fashion week proper thanks to a cash injection from the owners of Chanel and L'Oreal.
Paris will, however, be without Givenchy this time, with its new British designer Sarah Burton, a stalwart at Alexander McQueen for a quarter of a century, just made creative director.