VisualEditor should support scope designations for columns and rows in tables. This will significantly improve accessibility.
Just a quick note about the downside here: This markup, although officially recommended at enwiki, is almost never used at any wiki, even in Featured Articles (where it is nominally required). When this is implemented, I think we can reasonably expect experienced editors to be surprised, and therefore to complain about the visual mode adding "unnecessary" markup and creating dirty diffs every time a table is touched.
We should implement it anyway.
The choice of cells for markup for row headers and row scope is rather more problematical. Priority should be on adding scope for column headers as that should be easily achievable and much less likely to encounter opposition.
The other really significant addition would be for VisualEditor to encourage editors to add a caption by setting the default property to 'on', but whether that should be considered in this thread or separately is debatable.