I'm thinking that there should be several play modes which would affect how the game works. Note, these are still basic sketch ideas so comments are welcome, if not encouraged.
The first mode would allow multiple PacMen in the arena who face the ghost players. The PacMen will be against each other AND the ghosts, trying to eat the most dots. The powerups will be like normal powerpills, allowing a pacman to eat a ghost AND another PacMan. Non-powered Pacmen will not be able to pass each other, but will not come into harm if they collide. Ghosts will work co-operatively to take down the PacMen. Win conditions here would be obtain the most points; this would need some careful balancing of point scorings to be fair.
A second mode will be more like 'survival'... A minority of players will start as Ghosts and the rest will be PacMen. As the Ghosts come into contact with a PacMan player, the player will die, travel to the spawn point and return as a ghost. The aim here is to survive until the end, perhaps having to collect a certain number of points to finally 'win'.
I like superpig's 'killer' idea a lot. So, perhaps have a mode that features three teams - A, B and C. Team A would be hunters of B, but hunted by C. C would be hunted by B, but hunters of A and B would be hunted by A, but hunters of C. When one team is obliterated, the remaining two tams would be pitted against each other until one team is defeated.
As an arcade game, I think there should be the obligatory powerup system. These powerups should be able to affect speed, invulerability, invisibility, wall-passing ability and projectile firing. I think some 'dummy' powerups would also be good - one could rotate the hunting teams so that A would have to hunt C instead of B, for example. This would be a strategically interesting powerup, I think.
I might think this over some more, at least from a game design perspective. I like the interesting twist on what is, essentially, a played out game ;)