We are thinking of doubling that rate next month.
I'm not sure much can be done about the predominance of certain puzzle types in the very early Rush puzzles. Would you prefer more hung pieces?
But in the later levels, mixing it up by themes/tags is an interesting idea, about which I will make some measurements.
What is also pretty interesting would be a (fuzzed) per-tag Elo rating on the user - so they can see which themes you are good/bad at relatively (for example, if you constantly get problems on Trapped Pieces wrong your rating on those problems would be lower than your average Puzzle Rating).
+-------------------+------------+
| theme/tag | mdn_rating |
|-------------------+------------|
| Opposition | 1725 |
| Passed Pawns | 1494 |
| Rooks on Seventh | 1408 |
| Pawn Endgame | 1353 |
| ... | ... |
| Smothered Mate | 549 |
| Stalemate | 533 |
| Mate in 2 | 436 |
| Mate in 1 | 399 |
+-------------------+------------+
I also like the idea of showing the user's +/- for each theme. Though the data for some themes is better than data for others.If you produced fewer, would that increase the proportion with expert review?
https://www.chess.com/article/view/puzzle-rush-confirmed-rec... https://www.chess.com/news/view/new-puzzle-rush-chess
Although I don't think there were lots of players who tried to use this tactic among all the ones who occasionally train on chess.com.