I noticed that there is a great opportunity for the spec to support the abbr tag. While a thread in the past has asked to implement that in a way similar to Markdown Extra where you define the abbr tooltip after the text, that seems unwieldy since it would mark up EVERY mention of the keyword.
A better implementation would, I believe, be supported in a way that’s already part of the spec: the link syntax, minus the link.
<abbr title="Tooltip text">Text</abbr>
This works fluidly with the current spec since the part inside the quotations in the regular link syntax provides the title for the link. Since the only default function of the abbr tag is to provide a tooltip, it is essentially a link without a URL… which is exactly what this CommonMark syntax is. So it is a logical fit that naturally follows from the link syntax.