Former England captain Stewart, director of cricket at Surrey, questions the consequences if the 18 first-class counties rubber-stamp plans to pave the way for a lucrative eight-team city-based tournament by the end of this month.
In a BBC interview, he wonders if the impact of voting on "the unknown" may compromise the future for many of those asked by the England and Wales Cricket Board to agree to the constitutional amendment necessary to introduce a competition which will not involve all counties.
ECB chairman Colin Graves identified a "watershed moment" last week when he triggered the ballot to press ahead with the one-off amendment which the governing body hopes will ensure wider engagement with a new audience - as has been demonstrated by the success elsewhere of the Indian Premier League and Australia's Big Bash Twenty20 franchise tournaments.
Steward said: "I'm all for bettering things, but what is it going to look like?
"Even the ECB doesn't know."
Stewart added: "My concern, which I know a lot of other counties have - those who might not be able to speak about it because they are not in the same financial position - is that we are being asked to vote on something very soon, when we don't know exactly what we are voting for.
"To be asked to vote on an unknown is a bit dangerous.
"Is it being done for the good of all cricket, or is Test cricket, four-day cricket, 50-over cricket going to become second, third option or fourth option?"
Surrey have been one of a dwindling number of counties still sceptical about endorsing the new competition - although all have signed, along with the MCC, a media rights deed agreement to include it as a potential product to offer in future broadcast negotiations.
It is a moot point how persuasive it has been with cash-strapped counties that the ECB has guaranteed a £1.3million-a-year share of projected revenue to all who have signed up.
Stewart said: "There are a lot of counties in a terrible financial state. This £1.3m a year that helps them - is that enough to save them? I don't know."
Surrey is a different case, however, having secured capacity crowds at The Oval for many matches in the existing NatWest Blast - which would become the feeder event to the as-yet unnamed new Twenty20.
"We sell out our T20 home games, so from a purely blinkered Surrey view we would be happy to continue as we are," he said.
"But could the ECB have invested some of this money directly, before going the full hog with a new format, to try and help the financial problems some counties have found themselves in - those who have taken on massive debt to bring stadiums up to Test standard, for example?
"You have to move with the times, and I want this to work, but what damage might there be to the counties' history, too?"
Sussex are among those counties highly unlikely to find themselves hosting one of the eight teams in 2020.
Their chief executive Rob Andrew, formerly of the Rugby Football Union and an international star in that sport in his playing days, acknowledges the future is far from straightforward.
He is nonetheless optimistic, and told Sky Sports News: "I suspect in the next few months, never mind years, we will see some interesting times in cricket.
"We [Sussex] have been a little reticent certainly, and I think one of the main things is just protecting the future of Sussex.
"We've had some concerns over what it looks like, what does it mean for county cricket, what does it mean for a club like Sussex that is unlikely to host these matches.
"So we've just been asking for some assurances, some safeguards on what the future looks like for a county like Sussex ... but we're getting there, I think."
