What different types of themes are there in The Merchant of Venice?
There are certainly plenty you can pick from, as in all of Shakespeare's plays. You might choose to focus on love, marriage and friendship, justice and mercy or identity for example. However, I will focus on one of the main ones: money.
It is interesting that money is a theme of the love plot as well as the hate plot. Bassanio's need for money to pay court to Portia is of course what puts Antonio in the power of Shylock. Antonio and Shylock are very different merchants, yet all merchants try to make money. Jessica as well, when she elopes with Lorenzo, brings money with her, stealing the savings of her father. She then spends extravagantly what her father gained with such care and diligence. Attitudes to the getting and spending of money are a theme of the play. Wealth by itself does not bring Portia happiness. She needs to be loved and to give love. Spending money is more fun than getting it, and generosity is better than meanness. However, the actions of Jessica in exchanging a ring that her mother gave to her father for a monkey show her to be reckless, uncaring and impudent.
Contextually, Venice was a mercantile city, and its luxurious life was based on commerce, law and credit. Great wealth was made on the back of extremely risky merchant ventures that could win or lose millions and bring around tremendous reversal of fortunes, as in the case of Antonio at the end. Of course, Bassanio himself tries a risky venture, trying to wind the hand of Portia. He deserves Portia because he is not taken in by "outward shows" or the glamour of precious metals - he is granted moral insight. Portia's wealth is fabulous - she is the metaphorical Golden Fleece won by Jason. But Portia's readiness to give her fortune and herself away for love is matched by Antonio's. This loving and giving is in stark contrast to a greed which prefers ducats to a a daughter and a hate which prefers a pound of flesh to thousands of ducats.
Another interesting theme in the play is the parallel father/daughter relationships.
First, there is the relationship between Portia and her dead father. We can only guess at the the relationship based on the information in the play. It would seem that he loved his daughter very much. He knew that once he was dead, she would be besieged by all the eligible bachelors in the area. He also knew that this would be difficult even for his beautiful and intelligent daughter, so he helped her out. The caskets were a perfect device as well as the terms of picking one. Portia knew where her portrait was hidden and we are told that she would subtly manipulate an undesirable suitor to the wrong one. I would have to say that Portia's father knew, understood and especially loved his daughter very much.
The second father/daughter relationship in the play is Shylock and his daughter, Jessica. There is only one private scene with Shylock where he is home. All the other scenes in the play are in public and he must put on his public face. He must be Shylock the money lender. In Act II, scene 5, we see Shylock the father. He must go out to a business dinner but he would rather not go. It would appear that Shylock loves and trusts his daughter. He gives her the keys and tells her to lock the doors and be safe. There will be people out and he wants her to be safe. We don't know how long Shylock has been a single father. We do know that his life has not been easy and like all fathers, he wishes to make a more comfortable life for his daughter. He seems strict but in his world that is what he feels is necessary to protect his daughter. Unfortunately his protect is smoothering her and she rejects the world of her father.
Shakespeare gives us an interesting juxtaposition of father/daughter relationships in The Merchant of Venice.
Genre, as I have suggested somewhere, is a major theme in MV. Scholars note Marlowe's THE JEW OF MALTA and tell us it is farcical or black comedy. It seems that satire is everywhere an element or question in MV, Portia's "I have within my mind / a thousand raw tricks of these bragging Jacks, / Which I will practice"(3.4) is suggestive. In the opening lines we find "I am to learn"(1.1.5) and the word "teach" is found repeatedly through what follows. the repitition ends with the court scene. Therefore, while the play is comical it is also serious.
There a few different ones. Of course there is the obvious love theme, between Bassanio and Portia. There's also the Anti-Semetism theme i.e Christians against Jews. There's also the theme of commerce because there nearly everybody's relationship is based on them trading and having something to gain from one another. There's the bond theme between Antonio and Shylock; Bassanio and Antonio; Gratiano and Bassanio with Portia and Nerissa(wedding rings). I'm sure I'm missing some more but I hope this will get you thinking.