How does Mrs. Linde's character symbolize Nora's potential future in A Doll's House by Henrik Ibsen?

The character of Mrs. Linde is that of a woman who, after living her life, is now alone in her middle adulthood. She has had a tough life from the start. She had to give up the man she loved, Krogstad, and ended up marrying a businessman to take care of her mother and brothers. In this case, she sacrificed herself and the love of her life to support her family.

Years later, Mrs. Linde is completely alone. Even though she gave up everything, she did not gain anything from her sacrifice, much like the results of Nora's own sacrifice. Linde's mother and husband are dead, her brothers are all grown up, and she has lost whatever financial stability she once enjoyed. She is now at the mercy of whatever help she can get, and a huge part of this problem is not having the financial stability a man would provide in her society.

Mrs. Linde foreshadows Nora's future because Nora will be what Linde is now: a lonely woman living in a male-dominated society.

Both women tried their best to make something work for themselves by independently using resources to fix their problems. Those efforts led to helping others, but not themselves. Moreover, in the time and place where they live, women are nothing without men. Even Christine admits to as much when she asks Krogstad to take her back, saying,

Nils, how would it be if we two shipwrecked people could join forces? [. . . ] Two on the same piece of wreckage would stand a better chance than each on their own.

Hence, we may expect that Nora will also end up alone in her own middle adulthood. Like Mrs. Linde, she will be without children and leading a life that may depend on the goodwill and charity of others. Hopefully, we won't end up seeing Nora asking her husband to take her back for the sake of company and stability the way Linde does. If this is the case, it would be yet another way to show how Linde definitely represents Nora's future.