[Quoth]

Epub3 IMO isn't a later replacement for epub2, it's a different thing really for "interactive" titles on LCD/OLED.
POD needs a basic PDF, not any ebook format.

The only accessibility things that an ebook creator can do:
1) Flowing layout, never fixed layout.
2) No background
3) All body text black
4) Larger text for headings, but not too large
5) Check it's all fine on a monochrome screen with only 6 inbetween greys
6) No image based text, all text inc initial paragraph characters.
7) Possibly no drop or enlarged caps at a paragraph start, may look pretty but reduces compatibility and readability (even on paper).
8) Body text using a serif font. Chose any publisher font to ensure works on 150 dpi eink. Far the best for Dyslexics, no clear proof Open Dyslexic font helps. Avoid large blocks of bold or italic. Avoid too small text for notes etc.
9) Only footnotes in reference works, try an inline right justified paragraph in sans rather than linked footnotes.
10) Clarity is more important than pretty.
11) No animation or video.
12) No page numbers except as right margin "notes" as actual pages in a copy of a reference work that's only ever one paper layout (e.g. Hebrew Bible)

WCAG is irrelevant for books and ebooks. It's not even very good for web sites.
eBooks are ALREADY more accessible than printed paper due to user selected margins, font size, flow and ability for text to speech on some ereaders.
Almost all accessibility issues are to do with the ereader gadget & its software, unless the book is really bad.
Really bad evil stuff in an ebook:
A background (image or colour)
Non-black body text.
Text as images, but see exception below
Headings etc too large or colours / shade unreadible on eink
Animation/Video
Fixed layout

Only use small snippets of text as image and ONLY if the font/alphabet isn't likely to be supported, consider latin/roman english speaker transliteration in brackets after it. Most users of Latin/Roman font/alphabet can't read other systems even to pronounce it. Obviously if the book is Russian, Greek, Japanese, Arabic, Hebrew this doesn't apply, though Amazon STUPIDLY had maybe only Latin/Roman and some Greek letters originally on the Kindle, even though the underlying Linux supported most lanaguages years earlier. It had worse language support than 1988 DOS.