Just days before Mayor Bill de Blasio unveiled the backbone of his education agenda in 2015, the education department’s top strategist chimed in with a surprising suggestion.

“I’d like to propose that that we offer to include charter schools in our central enrollment system,” Josh Wallack wrote in an email to several other senior officials that September.

“It’s a nice ‘tip of the cap,’” Wallack said. “It also allows us to settle, once and for all, the question of whether the charters are operating with any selection bias because all families would use exactly the same admissions process. Equity. One system.”

Common — sometimes known as “universal” — enrollment systems exist in cities from Newark to Indianapolis. Backers of the approach argue it can simplify the often complex and time-intensive process required to apply to either district or charter schools in cities that allow parents to choose among both. Streamlining the process can put parents on equal footing instead of allowing those with more time, knowledge or resources from automatically getting a leg up.

Early in de Blasio’s administration, emails show, the idea was initially met with approval from other senior officials, including Karin Goldmark, an education advisor to de Blasio who now serves as a deputy chancellor responsible for overseeing partnerships with charter schools among other programs.

De Blasio’s aides quickly worked the idea into a draft of his 2015 speech, according to records released by City Hall in response to a lawsuit filed by NY1 and the New York Post. But standing in the auditorium of Bronx Latin just a few days later, the mayor made no mention of the potential overhaul of the city’s enrollment process, focusing instead on a slew of new initiatives known as his “equity and excellence” agenda.

Common enrollment systems have gained traction in recent years as some cities have embraced a “portfolio model” of schools, a new way of organizing school districts that has developed strong backing. This enrollment approach is central in New Orleans and Denver, which received input from Neil Dorosin, who created and once ran New York City’s high-school application system.

But unified enrollment also depends on a degree of trust and goodwill between district leaders and charter operators or advocates, which has sometimes appeared lacking in de Blasio’s New York.

That’s why the revelation de Blasio previously considered including charter schools in a common enrollment system is somewhat surprising. Even before de Blasio became mayor, he campaigned on the idea that charter schools have a “destructive impact” on traditional public schools.

And while he has at times softened that rhetoric and supported district-charter collaborations, the mayor recently said that charter schools should not be allowed to expand. (The state cap on the number of charter schools could prevent new ones from opening if the state legislature doesn’t act soon.) Pro-charter groups have also been unafraid to push back against his agenda.

Yet common enrollment can be seen as a kind of compromise, said Betheny Gross, who has studied the model in multiple cities and is a researcher with the Center for Reinventing Education.

“Everybody is asked to give something up,” Gross said. “Charters are asked to give up a lot of their direct control over enrollment, which is important to them because they get paid by the student.” Districts, meanwhile, have to cede the possibility that a simplified process may mean more students will consider and choose charters, taking per-pupil funding with them.

Under the plan briefly entertained by the de Blasio administration, families looking for an elementary school would have been able to select 12 options on a single application, regardless of whether the choice was a traditional district or charter school.

Families would have then been matched with their highest-ranked school that had space for them, as determined by a complex algorithm. Such systems also sometimes take into account other criteria, including siblings already attending the school or a certain proportion of low-income students. (Importantly, a unified application system does not necessarily mean all students can be admitted to a district school only via a lottery. Schools could conceivably still use screens or require students to clear academic bars for entry, which has led to intense academic and racial segregation among the city’s high schools.)

It’s unclear how seriously the administration considered the plan. Multiple charter leaders said the prospect of a unified enrollment system had been raised under the charter-friendly Bloomberg administration. But they did not have detailed conversations with de Blasio administration officials about the proposal.

“It was an idea that was floated before the speech, but it wasn’t the subject of a full policy analysis,” said Jaclyn Rothenberg, a de Blasio spokeswoman. “It obviously wasn’t the route City Hall eventually took.” She did not answer questions about whether the mayor is still considering the approach.

Any such plan would face logistical hurdles, including the need to revise state charter school laws. It’s also unclear whether the sector would line up behind a universal system, which would likely depend on charter schools opting in.

James Merriman, the CEO of the New York City Charter School Center, said there is “a lot of support from charter leaders” for a common enrollment plan. The Charter Center runs a separate common application system that includes most of the city’s charter schools, which educate over 100,000 students and is itself larger than Denver’s entire school system.

“We’d be very interested in seeing it,” Merriman said. “It’s embarrassing that New York City is behind in a process” that other cities have adopted. A spokeswoman for KIPP, which operates charter schools in several cities with common enrollment, said the network supports the idea in New York City.

Stacey Gauthier, principal of Renaissance Charter School in Queens, said a universal enrollment system could help charter schools avoid a common critique: that the city’s most vulnerable students may not know about the application process, or that schools dissuade certain students from enrolling. “It might take away some of the questions about who [charters] accept and who they don’t accept,” she said.

But Gauthier and others in the charter sector were less enthusiastic about the idea without first seeing all the elements of such a plan.

A spokeswoman for Success Academy, the city’s largest charter network, said their support would “depend on the details.”

And Derrell Bradford, executive director of the pro-charter group NYCAN, said it does not make sense to trust the de Blasio administration to take control of the enrollment process for charter schools, given its historic antagonism toward the sector.

You should never let your competitor be your regulator,” he said.