How about changing it so that:
- You can continue to do daily tests, even after 100, but only (the last) 100 count.
- Your credits from the daily tests gradually fades. Like after 1 year, a test is only worth 0.5 points, and after 2, 0.25 points. (Fading should probably stop about there.)
These would combine to make the daily tests an ongoing activity, instead of something you do for 4-6 months and then can't do any more. This could keep people's interest in he.net. (Which would be particularly good if you had news to show.)
--David