So let's brainstorm. I'm gonna put the current suggestions into two bags. One is "easy fixes". I call 'em easy because it's something where I can just change one value and upload. I don't need to make changes to the player-app or the rules-page or anything. . .
1. Limit the range of cars
2. Limit the range of zappers
3. Be more stingy in handing out new zappers and/or cars (i.e. not one of each every level)
4. Don't give out two of each at the onset.
And here's a list of stuff that'd be slightly more difficult. That is to say that I'd need to modify the game and the playback applet. Not that it's a problem. All of these fixes are fairly trivial.
5. Make the zappers take up a turn (i.e. the cats move after you use a zapper)
6. Make the number of cats grow more slowly, but with higher percentages of gray/orange cats
7. Make it so you automatically make a null-move if you don't move within a few seconds
8. Eventually eliminate houses entirely
Personally, I'm leaning towards number 6 and 8. IIRC, the proportion of gray/orange cats doesn't change (after level one, which doesn't have any), and later levels seem to be gummed up with so many brown cats that the orange/gray ones aren't as much of a problem as they should be, because they wipe themselves out in the first couple of turns. If you had a level consisting of 25 orange & gray cats (and no houses), you'd have a serious problem.
So basically things will still start out easy and beat-able, but it'll get difficult faster.