Twelfth Night would be an interesting play to examine from a feminist point of view. The play has three major female characters who affect the action in significant ways.
On one level, we have the story of Viola, whose father has died and whose brother she suspects is dead. Thus, when she is shipwrecked in Illyria and has no male protector, she disguises herself as a male as long as it is safe for her to do so. Eventually, when her masculinity causes her to get into some perilous situations, she retreats to her true feminine self and gains a husband and protector in Duke Orsino.
On another level, we have Olivia, who also has suffered from the death of male relatives, although her uncle Toby has arrived on the scene to watch over her (he doesn't seem to be much help, though, as he spends most of the play in an intoxicated state). Olivia may be a "helpless" female on the surface, but she seems to control her own destiny and drives Count Orsino to extreme frustration by refusing to marry him. Moreover, Olivia behaves more like a love-struck male once she experiences Cesario's (Viola in disguise) charms. Olivia strikes me as quite aggressive in her pursuit of Cesario. Ultimately, Olivia's marriage to Sebastian restores Olivia to her "proper" feminine status in Shakespeare's society.
A final powerful female in Twelfth Night is Maria. She's the brains behind the scheme to drive poor old Malvolio out of his wits. She functions like an internal director for the play as she directs Toby, Andrew, and Feste in their plot against Malvolio. Ultimately, even Maria is cast into a more traditional role as she is supposed to marry Sir Toby at the play's end.