Hah, I adore the playful cuttlefish idea. Playful critters in general are an area of personal affection for me.
The leaf/detritus/toy patch plant is easily enough done too; I love the idea of alternate uses for patch plants. And even though this is totally unintended, the patch plant system could technically be used to auto-spawn anything, including critters, so something like an aquamite patch might help to maintain aquatic food-chains.
Something I would really love to do is make a sort of ornamental fish that is composited from separate sprites, so say, the head, body, dorsal fin, ventral fin, and tail fin, would have tons of styles that would be randomly mixed and matched, built much like creatures are. That is totally outside my spriting ability right now, but would be a seriously fun project to code.
Said fish I'm thinking would be sort of modeled after betta fish-- with aggressive traits and a similar courtship ritual, so you couldn't just throw a bunch in the aquatic meso and see what interesting babies pop up; it would actually require some effort and human interaction to selectively breed pretty fish. I guess I'm just amused with the idea of using critters to essentially create mini-games within DS.
In general I think I'd like to work a lot more with 'evolving' plants and critters, not really in the same way creatures do, but more in line with the golden gnarlers-- just little things about the wildlife that change over long periods of time to sort of reward logging many hours of playtime in a single world.
And, this might be totally outside of my scope right now, but I'm toying with the idea of bringing a variation on C3's blueprints back into life for decoration agents, to allow people to save and share decoration setups that they've worked really hard on. It would basically take a snapshot of where all the decoration agents were and what state they were in, export it to a GB-agent file that, when re-injected, would place all those decorations exactly where they were, or in another area but with all the objects still placed relatively to each other. But whether or not I actually take that idea further will depend on whether or not world-decorating is actually considered worth doing by the community; I have a hunch it may have been one of those things I did purely for my own amusement
I'm not a developer myself but I've got a whole long list of ideas if you don't mind 