HTML validity and accessibility issues

I can spot some issues regarding the validity of HTML (like putting div inside span, or form inside h1) and also there is room for adding aria-* for better accessibility. Are you planning on a new design? If that's the case there will be huge changes on the markup I assume. That's why I am hesitating to fix the markup and accessibility cause that would cause HTML markup changes too. What do you suggest? I can also contribute to implementing the new design and do the validity/accessibility stuff along the way.