The design goals for Spare Squares include: easy enough that anyone who is the slightest bit interested (definitely not the majority of people at BGG.con) will feel that they have some prospect of winning even if they don't spend a great deal of time on the game; yet rich enough so that those people who enjoy fiddling with the cards will have fun spending a great deal of time on the game and earn some kind of reward for doing so. It's not easy to hit that balance.
An average random but legal arrangement of cards scores yields about 12 to 20 points along the outer edges. The maximum edge score is 36.
Here are some ideas for tweaking the rules:
1. Requirements: I am thinking of removing the requirement for any kind of interior edge matching. All you have to do is match the numbers. In practice, that will require trading with no more than two or three people. This should yield a set with about 16 to 20 points with a bit of rotation. Trading with a larger group, say 12 to 20 people, should yield a decent score of 24 to 30. Those last few points to get to 36 are the hardest.
2. Bonus points: Completing the first level bonus is about as hard as trading with two or three people (up to five, perhaps). That yields the average edge score of 12 to 20points plus the 10 points bonus, for 22 to 30 points. This automatically shuts out a casual number match and rotation submission. A better first level bonus is 6 points, putting you in the same category as the edge match and rotation submissions.
The second level bonus is as hard as the complete edge match. When combined with the average edge score (12 to 20), it should yield a little less than a perfect edge match (36) so as not to shut out edge matchers, so should be around 18. So 6/18 seems to be about right for bonus points.
Of course, if someone has access to hundreds of cards and lots of time (as they will), they're going to raise their scores above 36 and they will win, putting everyone else out of contention. This is a discouragement to the other players. On the one hand, isn't that the way games are? The person who practices more at Chess or Tennis than you is going to win. On the other hand, I don't have the luxury of only catering my game to fanatics; I have to be inclusive of lazy people.
3. To solve this, I could make each point an "entry" into a random draw. In this way, a casual submission counts as 16 entries if it is worth 16 points. A fanatic entry of 40 points is 40 entries, but still not a guaranteed win. This neatly solves the problem ... except that I hate lotteries. I may have to swallow my pride.
4. Another solution is to divide the field up according to play tiers: one prize for sets with no matching middles, one prize for sets with first tier matching, and one prize for sets with strong (perfect) matching. Fewer people will play in the strong match tier, and one of them will get their prize. Many people (one hopes) will play in the no matching middle tier, and the winner can be chosen randomly among top scorers of this tier.
This option still requires the number matching for all tiers, however prizes are not given out based on the number, but based on the tier (three prizes instead of four).
I'm still thinking about it. Any other suggestions (that don't involve changing the entire concept of the game, or the design of the cards) are welcome.