Well , here are some possibilities, and my two cents on them .
Gwen Stacy/ Spider-Gwen
One of Peter's great loves , whose been adapted twice before on screen.
However , Marvel studios may want avoid adapting her a third time, and in the case of Spider Gwen,
Sony may not give them permission to adapt a live action version of that character, instead
saving her for their own live action adaptation, down the line
Black Cat / Felica Hardy
A front runner imo .
They 've attempted to bring her to the big screen as far back as Spiderman 2.
Felicia appeared in ASM2 , but never made it to the suit.
However, Sony may still be unwilling to allow Marvel to use the character due to
their on again off again development of Silver and Black .

Carlie Cooper and Lilly Hollister
Two newer characters to the Spiderman myth , but are strong possibilities give The
Brand New Day title.
Either one of them would certainly fit in as apart of Peter's
new supporting cast .
On the other hand, Marvel and Sony may want a character more well known and prominent, from the various Spiderman films, and tv shows.
Jackpot / Multiverse Mary Jane
This is among the least likely options imo
Zendaya ,
is The MJ for the MCU Spiderman , and I'm skeptical Sony and Marvel would now introduce Mary Jane Watson as " the real MJ" for Holland's Peter.
Mayday Parker
While interesting as a fan theory, I also doubt she's playing the daughter of Tobey and Kirsten's
Peter and MJ .

Firestar / Anjelica Jones or Jean Grey
My guess is that both of these X Men related characters will be off the table for a Sony film.
They're both redheads , but that doesn't mean Sadie is playing either on of them, or that Sadie's hair will be red,
Spider-Man : Brand New Day.
A New Character created for the film
A brand new character ,created just for the film, is the least likely option imo.
Even though there's precedence for creating a love interest specifically for a film ( Chase Meridian ,
Batman Forever) , nowadays, studios tend to use existing characters, they have the rights to.
So while they could , in theory ,create a whole new character / love interest, my guess is that Sony will utilize an existing character as opposed to creating someone brand new for Sadie Sink to play.