Unwritten rule with Supreme Court Justices seems to be the replacement is always of same gender. Also their education because all 8 current Justices have either Yale or Harvard background and this same thing also applied to RBG. President Trump's Supreme Court nominations list includes 12 female candidates:

Allison Jones Rushing: Wake Forest University (BA) Duke University (JD)

Joan Larsen: University of Northern Iowa (BA) Northwestern University (JD)

Amy Coney Barrett: Rhodes College (BA) University of Notre Dame (JD)

Diane S. Sykes: Northwestern University (BS) Marquette University Law School (JD)

Bridget Shelton Bade: Arizona State University (BA, JD)

Allison H. Eid: Stanford University (BA) University of Chicago (JD)

Britt Grant: Wake Forest University (BA) Stanford University (JD)

Barbara Lagoa: Florida International University (BA) Columbia University (JD)

Margaret A. Ryan: Knox College (BA) Notre Dame Law School (JD)

Martha M. Pacold: Indiana University (BA) University of Chicago Law School (JD)

Sarah Pitlyk: Boston College (BA) Georgetown University (MA) Katholieke Universiteit Leuven (MA) Yale Law School (JD)

Kate Comerford Todd: Cornell University (BA) Harvard Law School (JD)

If we focus only on which candidates have either Yale or Harvard background, it leaves us with Sarah Pitlyk and Kate comerford Todd. As a sidenote last 2 Justices(Gorsuch and Kavanaugh) have interesting connection with eachothers. Kavanaugh clerked for Supreme Court Justice Anthony Kennedy from 1993–1994, working alongside fellow high school alumnus Neil Gorsuch. Gorsuch also clerked for Supreme Court Justice Byron White during this same period. So what about Sarah and Kate? Kate was co-clerk for Justice Clarence Thomas while Sarah was clerking for Justice Brett Kavanaugh so both of them have clerk experience with either former or current Justices. I would say overall Sarah is more qualified when it comes to education and overall experience. If Trump is willing to choose someone outside of Yale/Harvard background, Amy Coney Barrett is also one likely option. She too has connection to Justice Antonin Scalia whom she clerked for 1 year.

https://en.wikipedia.org/wiki/Donald_Trump_Supreme_Court_candidates

https://en.wikipedia.org/wiki/Neil_Gorsuch

https://en.wikipedia.org/wiki/Brett_Kavanaugh

https://en.wikipedia.org/wiki/Sarah_Pitlyk

https://en.wikipedia.org/wiki/Kate_Comerford_Todd

https://en.wikipedia.org/wiki/Amy_Coney_Barrett