Other than the difficulty of love, what is one other major theme in "A Midsummer Night's Dream"?
How about the theme of translation and transformation? The play is full of the idea that things change, that things change from one thing into another.
"Bless thee, Bottom, bless thee! Thou art translated", Quince shouts to him after he's been transformed into a donkey by Puck. And, here's Helena, saying she wants to be transformed into Hermia in Act 1, Scene 1:
Were the world mine, Demetrius being bated,
The rest I'd give to be to you translated.
And in Helena's soliloquy, she argues that
Things base and vile, folding no quantity,
Love can transpose to form and dignity.
The play makes this argument quite clear. The flower "love-in-idleness" transforms lover into hater, hater into lover, and eventually, lover back into what they were before. Love transforms.
The mechanicals play-within-a-play also depends on acting, which itself is a sort of transformation: one person (Bottom) becoming (Pyramus). Is there a moment, as is often claimed, at the end of the play where Flute believably becomes Thisbe?
Moreover, many directors (beginning with Brook in 1970) have decided that the play translates Theseus and Hippolyta into Oberon and Titania, and Philostrate into Puck. Perhaps the whole play is a sort of transformation...
"Deception", or "trickery" is another theme. From the first act when Lysander comes up with the plan for him and Hermia to elope to the end and Puck's final speech to the audience, we see examples of deception. The mix-ups with the flower's nectar causing Lysander to think he loved Helena and causing Titania to think she loved Bottom are all examples of deception. When Puck gave Bottom the head of an ass, Bottom never realized it - he was deceived. Oberon, in order to trick Titania into giving up the boy she is caring for, tricks Titania with Puck's help. Helena is sure that Demetrius and Lysander, along with Hermia, are simply playing a mean trick on her when both Lysander and Demetrius claim to be in love with Helena.