The England and Wales Cricket Board have proposed a 100-balls-a-side format for a new domestic competition that will start in 2020.
It had been thought that the eight team men's and women's competitions, due to involve city-based sides rather than the traditional 18 first-class counties, would have a standard 20-overs per side format.
But English men's county cricket, which pioneered the 20-over game as a professional format, already has the Twenty20 Blast, while the Women's Super League is also a 20 overs per side competition.
Instead, in a bid to make the new tournament "distinct", the ECB has now opted for an unproven format that would, if adopted, differentiate the event from existing Twenty20 franchise competitions such as the Indian Premier League and the Caribbean Premier League, which both currently cut across the English season.
An ECB statement issued on Thursday said the competitions would take place in a five-week block in the middle of the season.
Under the proposal, which has still to gain final approval, each team will face 15 standard six-ball overs with an additional 10-ball over at the end of the innings.
That would mean the tournament deviating from cricket's Law 17.1 which clearly states: "The ball shall be bowled from each end alternately in overs of six balls."
The plan was put to the chairmen and chief executives of the first-class counties and MCC, the owners of Lord's Cricket Ground, by the ECB on Thursday.
"This is a fresh and exciting idea which will appeal to a younger audience and attract new fans to the game," said ECB chief executive Tom Harrison.
"Throughout its development, we have shown leadership, provided challenge and followed a process. We will continue to do that as the concept evolves."
Meanwhile England paceman Stuart Broad said on Thursday he was in favour of the new format.
"I'm hugely optimistic, I love the fact that it's different to all the other tournaments worldwide -- 15 six-ball overs and then the pressure of a 10-ball over to finish," Broad told Sky Sports.
Asked how he would feel about bowling the final 10-ball over, Broad added: "I think I'd prefer the first set! You're stepping into the unknown a little bit because that tactical side of the game hasn't been used anywhere in the world."