There are several excellent suggestions in the Enotes page, "Topics for Futher Study," excerpted here:

1)Compare and contrast Andrew Crocker-Harris with Willy Loman from Arthur Miller’s Death of a Salesman (1949). Both characters are trapped in unhappy situations. How do they handle the problems in their lives?

2) How could Millie and Andrew have avoided their unhappy situation? Was the end of their marriage inevitable? Discuss how certain actions— better communication, compromise, marriage counseling—could have impacted their relationship.

3) Compare and contrast Andrew Crocker-Harris with Mr. Chips, the protagonist of the movie Goodbye Mr. Chips (1939). This movie concerns the life of a British schoolmaster, Mr. Chips. How do these characters regard their positions? How does this attitude affect those around them, including students and family?

4) Research the psychology of wives who cheat on their husbands. How do Millie’s actions fit into your findings? Do you believe Millie and Frank really love each other?

As for the "eternal love triangle," literature is rife with such themes.  In Shakespeare, you could use "Much Ado About Nothing," for example, or for a more modern approach, "The Great Gatsby."

If you wanted to take a historical approach, consider teaching the play in conjunction with the events that led up to the Cold War and the fall of the Berlin Wall.