Wow! These are really cool suggestions, so I'll respond to them individually
> Richard Scarry's busy little town illustrations and Where's Wally books
This is a cool idea, but I want to avoid it because I want to have a lot more frogs and sharks. Currently it takes me about 5 minutes to draw a new frog, so I could probably add like 50 frogs in a day if I really went at it. ...This suggestion makes it harder to add new frogs because at a mininum, I'd have to have new graphics for each frog without the 'card' outline, and logic for where each frog belongs. But to actually make it look good -- I'd probably want to have logic to make sure you don't drop a scuba diving frog in the sand, or a weighlifting frog in the middle of the sky. So I'd have to choose between having 100-200 simple frogs like I have now, or 20-30 cool Richard Scarry frogs.
> It'd make sense to have a place (a separate scene maybe?) for sharks too
I love this idea. I have ideas for a frog artist who draws you pretty frog pictures as you find more frogs. Maybe there can be a shark artist too!
> I'm a big fan of inviting players to reflect rather than compete
I greatly respect your ideas on this. It's discouraging to repeatedly look at a high score you set months ago, or to be faced with the implication that every playthrough is pointless if you didn't set a record. A "recent N playthroughs" idea is intriguing but seems like overkill when there are 20-30 different levels to chart, so I'd lean towards something simpler
> additional UI responsiveness - hover sounds, maybe visual pops
These are all good ideas! I was originally going to have the cards flip over like playing cards, although now I like the snappiness and surprise of just having them "Pop". I still think a little squash stretch or scaling would be a good addition. I don't want to overdo it because I think the understated subtlety of the UI adds to its charm -- but just a little!
> I'd love to see the "progress summary" screens be interactive... I'd also love to see a "return to menu" option
I think both of these could go together. Maybe a "bye bye" button in the bottom left to return to the main menu, and a "fast forward" button in the bottom right. The fast forward button could skip to the next level -- or if you found a shark, the fast forward button could spawn more sharks and make them run faster.
> if the sharks had a small chance to trip
Ha ha, this is a very cute idea. ...I don't think it would be hard to add. ...Actually, when I was first making the game, I considered letting the player keep their finger if they avoid sharks for long enough. But, it feels like it might turn what's currently a cute easter egg into a tedious mandatory experience.
Thanks for all your suggestions! I'm going to turn most of these into change requests and enter them into my backlog for the final game. They're very good!!