I had an idea:
Basically, you are tossed in with a bunch of random players who also pay to participate, and in the event, you fight wave after wave of random monsters (excluding stationary ones such as turrets), and for each wave you survive, every living person gets some money or items as a reward. Once you die, it kicks you out. The enemies would also occasionally drop hearts and etc. Basically, a wave-by-wave fighting thing with maybe a boss at the end and maybe an ultimate reward for anyone who survives the whole thing?
Kinda sounds like normal gameplay, especially arena levels.