After a certain amount of time, the map area decreases 20 blocks at a time, so if you are within 20 blocks of the glass when the certain time comes, you fall into the void including as the blocks you were standing on, (from 20 blocks of dome)
Another idea is that if you don't fight someone after certain amount of time, or you are in a region for xx amount of time, then you begin dropping/losing random things in your inventory.
My final idea is a flood, so when the time has come, the map gets filled nearly to the top with water, with limited air pockets, and everyone can see each other, if you don't make it in time, lily pads fill the top surface, leaving you to drown...
Hoped you liked my ideas (I haven't read all of them, so someone might have said what i said before me)