I suggest a consequence of events something as follows:
Titania decides to kidnap Juliet in the ball because she's so pretty and charming. She successfully does so, possibly noticed by the other fairies but not stopped by them (Oberon may try if he wants, but hopefully shouldn't succeed).
There is much commotion when people notice Juliet has disappeared, Tybalt probably blaming Montagues on it and goodness knows what everyone else will think.
Somehow (perhaps with Puck's help if he wants to stir mischief) both Romeo and Tybalt find out that Juliet has probably been taken to the forest. They decide to rush in there to save her - both at the same time.
Mercutio and Benvolio notice this before long and rush after them, declaring that they won't let Romeo and Tybalt go off into the forest like that but will join them to make sure they won't kill each other. (This is probably wise. Of course, if the Oberon/Benvolio thing has already started happening, Benvolio may have his own vested interests. Mercutio may have his own interests with regard to Tybalt. And so on.)
And voilà, we have a somewhat unlikely group of four together in the forest at night, looking for Juliet, and much mischief can be managed.
Before this, Lysander and Hermia have probably already gone off into the forest (and may be scared by the sounds of people looking for Juliet, thinking it's Hermia's family looking for her). Possibly pursued by Demetrius and Helena as in the original story; there's probably no reason why we shouldn't go that route.
Sounds like a good recipe for lots of chaos, right? And then we can look for reasons for the groups in the forest to break up and run into other people and whatever, and Puck can use magic juice or romances can get convoluted quite on their own.
Feel free to say your opinion or make suggestions!