Helena would be a tough character to direct in any version, modern or not. The reason for that is because her emotional state is a bit bi-polar. On one hand, Helena truly believes in love. She wants love. She is in love with Demetrius. On the other hand, she is quite jaded with love. Demetrius doesn't reciprocate her feelings, because he is in love with Hermia. That causes some self-doubt for Helena. She worries about her appearance and never quite believes Lysander is truthful about his love for her.
So to direct her I would want an actress that can pull off that longing infatuation over a man. She has to convince the audience that she loves Demetrius every time that she looks at him. But the actress also needs to have a bit of biting sarcasm that sells her jaded attitude toward men and their inability to see the lover that is in front of them. "Love looks not with the eyes..." she says of Demetrius, basically saying that he can't see what she can offer.
I really hate to say it, but Kristen Stewart's Bella Swan does a nice job of displaying her complete infatuation with Edward while at the same time being jaded toward the world. In fact, despite Bella being wildly in love with Edward, her lack of self confidence causes her to doubt Edward's affections for her. She can't believe that someone like Edward could possibly love her in return. I'm not a fan of Twilight or Kristen Stewart, but I have to admit that she might be able to pull off Helena. Especially if you told her to act how Bella Swan would act.
I have directed this play, as well as scenes from it, many, many times, and I have always loved the role of Helena. She is one of the quartet of passion-dominated, romantic lovers in the play, which includes Hermia, Lysander and Demetrius.
First of all, in casting the part, it is important to cast a young woman who is able to understand and convey intelligence overwhelmed by passion, and who understands that comedy is achieved through her inability to behave rationally when dominated by her irrational compulsion. I would direct her to imagine that, in order to function, she MUST be near her love object, Demetrius, no matter the cost. I would ask her to think of only one thing as she goes about her actions in the play, and that would be to be near Demetrius. She is single-minded in her focus, which lends itself to comic situations, and I would encourage her, in every scene, to focus on how to achieve her ONE desire...to be near Demetrius and to win back his love.
This would be the approach to Helena no matter what time period is chosen for the production.