hmm... since in HTML5 you are forced to use id attribute of some container, there is a risk of duplicate id.
Let's say Your plugin adds the id's to every Hx tag with its "safenamed" name like this:
<h2 id="my_amazing_title_">My Amazing TITLE!</h2>
OK, but what in case the page contains a heading like "calendar" and somewhere in the page there is also some
<div id="calendar">...</div>... hups! Now we are having a duplicate id... that's not good.
[ Signature moderated. ]