Sigourney Weaver is the latest name to be linked to The Expendabelles, after Sylvester Stallone teased she could play his screen wife.
The female-led spin-off of The Expendables has been given the green light, with shooting planned for 2015.
Sly, who returns to play gang leader Barney Ross in the third instalment of the action franchise, said they still have some issues to iron out with The Expendabelles as he promoted The Expendables 3 in London.
"With The Expendabelles, you've got a situation here where we're in uncharted waters. To put all women together, would that really work?" he said.
"You have to maybe put women who are really known to be tough, like female fighters... and are the Expendabelles really part of a divorce with Barney?
The 68-year-old actor and director continued: "If Sigourney Weaver was my wife, she could get half of the Expendables so he would have to deal with that too. He's lost his house and his mercenaries. It's all these things so when we do it, it doesn't just languish there and use the (Expendables) name. It has to be something that would hold its own."
Expendables producer Avi Lerner revealed: "We are right in the final stages of the script. We have lots of ideas about who is going to be the action movie stars and we plan to start shooting next year. That's all I can say."
Australian filmmaker Robert Luketic has signed up to direct the spin-off, which would focus on "an elite group of highly-trained female mercenaries" on a covert hostage rescue mission.
Actresses such as Meryl Streep, Cameron Diaz and Milla Jovovich have previously been linked to roles.