Chadha, who has lost 25 kg, has already spoken to Naseeruddin Shah to play Muhammad Ali Jinnah in the film -- and he's game.
"This is my biggest challenge to date. I've been working on my partition film for some time now. The central character is Lord Mountbatten. The movie is set in January 1947 when Mountbatten was asked to oversee India and the movie ends in August 1947, so it's a brief time span.
"But (it is) a huge canvas requiring tonnes and tonnes of research and scores of characters. I've spoken to some people including Naseer and Saif. Nehru's role is a cameo, so I wonder if Saif would be willing to do it," she said on phone from London.
She has been working on the partition film while changing nappies and feeding the babies. She plans to roll it on floors in October.
The film's central figure would be Lord Mountbatten, and either of the two Hollywood biggies -- Colin Firth or Michael Fassbender - will step in to play the role.
The "Bend It Like Beckham" and "Bride and Prejudice" filmmaker has already locked locations for the shooting.
"I've already acquired the necessary permission to shoot everywhere in India including Rashtrapati Bhavan. So I am all set. I don't have to shoot in Pakistan at all. This would be my ultimate dream project and I can't wait for it to take off," she added.
But she has to wait until October because an astrologer has advised her so.
"The wife of the author of the book, on which my partition film is based, is heavily into astrology. She says October is a good time to start," she said.
For now, she is looking forward to running between Blackpool and Manchester with the Olympics torch Saturday.
"Though it's just a 300-yard run, I've been practising for it for months. There will be 8,000 people to cheer me on. My friends and relatives are coming there with drums and other musical instruments to cheer me.
"I am super excited to have been selected to run and I see it as a great honour. I get to keep the torch too, so my children will have a very special memento to keep in years to come," added the 52-year-old.
Since her five-year-old twins have started attending school now, she has six hours to herself at home.
"I'm making up for lost time," she quipped.
Chadha was also in Mumbai last month for her friend Karan Johar's 40th birthday party.
"But no one saw me slip in and slip out, probably because there was so much less of me now to see," she quipped.