Sometimes it pays to look for clues and information on official Nintendo game pages published for the Japanese market. Some eagle-eyed Mario Maker fans over on NeoGaf have been looking around the Japanese website for the game and found some interesting pointers on potential content.
For starters, as you can see in the graphic below Yoshi is stood rather prominently, suggesting the faithful companion may be included in the title.
Secondly, looking at the panning anniversary graphic on this page has some fans fantasizing about the potential presence of Kuribo's Shoe as an item, as it's included (along with Yoshi and many other features) as part of the celebratory image. That's a lot more speculative (though both instances are speculative) than the suggestion of Yoshi being included, admittedly.
In any case, Yoshi seems highly likely as an inclusion. Of course, the nature of the game should suggest that all items and characters will be featured across the four visual styles confirmed for the game - Super Mario Bros., Super Mario Bros. 3, Super Mario World and New Super Mario Bros. U. Yoshi would be a minor headache on that score, as would items such as Kuribo's shoe. Perhaps these considerations - and a tie-in with the 30th Anniversary - are part of the reasoning for this one now targeting September, following previous plans for it to arrive in the first half of 2015.
How much would you like Yoshi to be part of Mario Maker, and even Kuribo's shoe? Neither is confirmed, but lt us know in the comments regardless.