That would be nice.
Except for the fact that Yoshi's Island is fundamentally different from the mainline Mario series, and would therefore require it's own Maker engine, unlike the Mario games which are all fundamentally similar and only need to have different textures and some different blocks between styles.
I'm personally still waiting for Hyrule Maker.
Okay.
Except for that Yoshi's Island level design is still structured in a block based system even if the terrain has plenty of sloping features. Luckily Yoshi can still junp like Mario can, and getting Baby Mario is a mechanic of the game that shouldn't be too hard to get in. You still need to get from the start to the finish by platforming; all the mechanics fall into place.
Sure the core gameplay is fundamentally different, which is why it will be packaged and sold independently as DLC. I will disagree that it will require a new engine, for the platforming already exists. Replace the old enemies with Shy Guys, Crazee Dayzees, and the like, add in Yoshi blocks to get eggs to throw, and you have yourself a pretty substantial DLC package that differs itself away from the current Mario platform-to-goal style we have come to know and love.
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...even if Yoshi still needs to platform-to-goal, which follows the exact same layout as Mario's objective. Just with new, mechanics and gameplay changes.