Pronunciation / Idioms

I’m not sure exactly what you mean. The way you handle pronunciation and idiom descriptions in content is to add a textual explanation, such as a phonetic explanation of pronunciation or description of idiom. I’m not sure there’s any meaningful way that WP Accessibility could do any part of that.

Yes, textual explanation is a think, which could be covered by your plugin, because there′s still no nice solution, where I can set these explanations in the editor for each word (except directly behind the word). An explanation could look like:
You see a word linked. When you click on the word a box with the explanation appears.

For that, what you’d really need is something like a glossary plug-in. Yes, WP Accessibility could incorporate a glossary and provide you with an interface to define terms, but that could get tricky. It’s an interesting thought, however. I’ve written a glossary plug-in before that filtered content to insert links to definitions; this would be a moderately more complex version of that. I’ll think about it.