Your Tribalstack Tropics character ideas fit the world nicely. They could also have had a bug, like a scarab, or something to represent the tribespeople that isn't just cosmetic changes to the corplets. I mean, YL isn't only lackluster when it comes to characters to talk with, but the enemies are also really unimaginative (with maybe one or two exceptions) and repetitive. So basically anything would work.
In the casino, you could have rich moles with monocles and top hats, red racing cars, chips and dice jumping around, bags of money, a roulette ball that you need to bring back to work, egyptian statues... I'm making these up as I go, but tbh I think anything's better than "the same character thrice".
If I do the same for MMM, how about will o wisps, crocodiles, big fireflies, old garbage like boots and stuff, a worm sitting on a fishing hook and not coming down because he's scared of heights, a giant fish, crabs, ghosts, little rusty robots that fall apart when you hit them and reassemble later...
The ice level could use color more than any other one, so how about one or two castle supplies, like lighters or armor coming to life. Old rare would probably have placed a giant snowflake with eyes somewhere, too. I think they are very rarely doing that in YL (just attatching eyes to stuff), but I felt that always worked nicely. Maybe because of that one opponent? Also, maybe bats and a yeti in the caves?
Galleon Galaxy is mostly okay, but a few more aliens wouldn't have hurt. It's easy to go wild here. Little things in tiny saucers, goo monsters, something living in the fog. And robots, or junk pirates. Little guys with a lot of military insignias to show they're some kind of space major. Some kind of shapeshifter that disguises itself as an item and transforms when you get closer.