how about...
2 points for attending a game (swiping in), 1 point for leaving after the full-time (tap out on exit).
-3 points for missing a game (not swiping in), no penalty if you upload to ticket exchange for your first 5 games, if you upload to ticket exchange for more than 5 games you'll only get 1 point back (so lose 2), as you're missing too many games.
You will also get bonus points for attending a sequence of games, so if you attend 5 games in a row, you get 5 extra points, 10 games, 10 points etc. You could also do bonus points for cup schemes and midweek games.
If you end up with a positive points score, you get those points as a discount against your season card. So you attend all 19 games, you get £57 deducted off your season ticket. (19 x 2 points plus a 19 game sequence).
If you end up with a negative score, then the penalties could range from being frozen out from purchasing away tickets for a season, an increase on your seasoncard price etc. That way you're incentivised to have your card used and your seat filled, and you have to pay more to keep it empty.