- Letter Blocks This is an interesting idea! There's a lot of potential for hardware that sits on or around the iPad - this is basically what Darren's stuff does, and Osmo.
I am inspired by some of the early toys/games in this space (this is probably the best example: https://www.google.com/search?q=cars+ipad+game&oq=cars+ipad+game&aqs=chrome..69i57j0l5.2178j0j4&sourceid=chrome&ie=UTF-8). There hasn't been much followup work in this space, but it's relatively easy to add conductive tape or whatever onto a block and track it with the iPad.
What I hear in in the student feedback is partly a need for more interesting/engaging activities, regardless of the modality.
- Braille Piano. Seems like it could be fun. I'm always a little skeptical about combining screen readers and music - partly because a lot of the accessible block programming tools that aren't StoryBlocks focus on music - but end up being a lousier programming language than Scratch, and also a lousy musical instrument. Slightly before your time