#1. Battle Royale
This is a structure pulled from wrestling. Put a large number of characters in a room or setting. All the characters want to be the only person in the room. Create a conflict where characters are ousted from the room by the others. The play is over when there is only one character left in the room free to do the thing that he desperately wanted to do alone.
#2. Royal Rumble
This is just like the previous prompt except this time only two people start in the room. Each of them wants to be alone in the room for some secret reason. But the only problem is that a whole bunch of other people need to be in the room for some other reason also. So at regular intervals in the play someone enters and is trying to get everyone else out of the room. Again the play ends when only one person is left.
Some things that these two prompts teach you about is being flexible about exposition (why do people need to be in this room?) and character motivations and secrets (what are they willing to do to be in this room alone? What is their reward? Also explore different characters creating alliances and stabbing each other in the back. These things are little triggers in every plot that can move the story along.
An Onion is an old Theatre Improv game but it can work find for a play. One character starts the first scene. When you get bored or have no idea where else the character can go bring another character in. When they enter the scene changes to something completely different. When you keep getting a block add another character! When you've reached the max number of characters you think your play will contain start taking characters out of the scenes. The play is over when there's only one character left in the scene again.