What a wonderful idea for an assignment. We are left to wonder what happens to everyone after the door slams.
Torvald is not a particularly well-developed character, so it will require a lot of introspection to figure out what makes him tick and to identify what his logical next steps would be.
Torvald is a very conventional person and a bit of an ostrich. He wanted to believe his life with Nora was moving along according to plan, and it seems that he willingly overlooked the signs that something was amiss.
Several key questions to consider:
Will Torvald's behavior actually change? Does he truly want Nora back? If so, his monologue could largely consist of possible plans to get her back.
Is Torvald willing to accept responsibility, or will he continue to blame Nora? If he is still undecided, his speech could include more of his interior states as he ponders how things got to this point.
Also, what play might this speech be part of? Is it in one more act added onto the existing play? The first act of a new play? To what broader action might this one character's speech be connected?
In addition, remember to take clues from his lines in the play. Torvald speaks of his having "cherished" Nora, so we must ask if he believes this. But he also speaks of how things look "in the eyes of the world"—would that remain a more important concern than repairing his marriage?
Finally, it might be fun to take things in a different direction. Maybe Torvald wants a complete break with this tainted past. Thousands of Norwegians went to the New World in the nineteenth century—maybe Torvald would like start a new life in, say, Minnesota.
Lucas Hnath is among those who wondered what happened, and in 2017 he wrote a play following up with Nora and company fifteen years later. It is called simply A Doll's House, Part 2.