One way to invent interesting new kinds of video game is to take an existing video game, and invert it, in the sense of having the player take the part of what was previously the bad guy or monster or antagonistic force of nature. Perhaps the clearest example of this was Dungeon Keeper (1997), which inverts the general D&D theme of a party of adventurers wading into a dungeon and hacking and slaying; now you're trying to keep the dungeon in good order and these pesky adventurers keep coming in and making trouble.

I was recently idly wondering whether any of the real golden-oldie games could usefully be inverted and hadn't yet been. Space Invaders could just about be, for example: it's fundamental to the nature of the game that the invaders move in a fixed pattern, but you could control their firing by clicking the mouse on an invader to have it drop a bomb, and your aim would be to try to fire bombs in the right pattern to box the defending ship into a killing zone. That doesn't sound like a particularly interesting game – certainly not interesting enough to motivate me to sit down and write it – but it illustrates the point. Qix might be a more interesting one to invert, on the other hand; and I've actually seen a quite playable inversion of Asteroids.

Another one I was idly wondering about the other day was Pac-Man: there surely, I thought, must be scope for a game with a slightly RTS-like interface by which one player controls all four ghosts and tries to box in the computer-controlled Pac-Man?

But just now I realised why it wouldn't work: there is in fact a trivial strategy by which four intelligently cooperating ghosts can guarantee never to let Pac-Man finish a level. Each ghost moves directly to one of the four power pills, and sits on it. When Pac-Man comes near that pill, the ghost moves one step towards the direction he's coming from, so as to cover the adjacent dot. If Pac-Man circles round and comes at the pill from the other direction, the ghost has ample time to move two steps back and cover the other adjacent dot. Hence, each ghost can reliably protect the power pill and the two dots next to it, and Pac-Man will eventually run out of other dots to eat.

(I suppose if you adjusted the winning condition so that the ghosts had to kill Pac-Man rather than merely preventing him reaching the next level, that might become more interesting again.)