Your OW looks a bit blocky. Most people don't like blocky although I, personally, think it looks pretty good. You also have a typo in your message box "blue koopa grew".

Looks like a riot to play, one question though. For 'Super Koopa' will the main koopa get a shell or a different colour? or some other form of notification? Or are you going to use a health style game?

Yoy might want to fix the overworld pallets for the koopa in the status bar and the walking koopa icon.

sorry about the incomprehensible message, it was quite late here. When I say 'health-style' I mean useing a health bar such as in 'Mario and The Island Of Mystery' the creator used a Link-esqe heart system.

PLOT:

Bowser has stolen Yoshi's eggs so yoshi sets off to find them. HiS friend Koopa knows yoshi wont get far and probably get trapped in an egg so he also sets off to find the eggs. When the word gets out Koopa is looking for the eggs a group of bullies all of the land fear make plans to stop Koopa, they are the....

BLUE KOOPA CREW!

Um... This may be nit-picking a bit, but why is Koopa friends with Yoshi? Aren't they enemies?


As a suggestion though, Your second world has a muncher theme, right?
Well, I would suggest incorporating Munchers into level design in that world. Nothing really obtrusive, but add them in to the level design to make the player have to go through some more obstacles.

For example, Maybe in one level, Mario will have to find a Key. That key could be on the other side of a ditch populated by munchers. Then, Mario would need to find an alternate route to the Key.