Currently it's possible to format text with <del> by using the syntax ~~foobar~~: foobar. This is quite useful for representing removed text and inline diffs. However, it should also be possible to format text with the reciprocal element, <ins> (to represent inserted text). This is particularly relevant for a markup language meant to be used in a code review context.
Markup for this could be e.g. ++foobar++, which is currently unused: ++foobar++.
Note: the <ins> tag is typically rendered as underlined text, so this would be very similar to the <u> formatting that is produced with __foobar__ → foobar. In fact, it might be worth repurposing that rule to produce <ins> instead, since ~~foobar~~ actually creates a semantic <del> tag rather than the presentational equivalent <s> (strikethrough) tag.
Doing so would make the behavior more consistent overall. For example, even **foobar** and //foobar// already produce the semantic tags <strong> and <em> instead of the presentational <b> and <i>.
If the goal is to make it denote "inserted text", I think it should be visually different to "underline" - something more similar to the green we use for actual diff views, maybe more subtle. Will need special handling in the color-blindness themes.
I don't normally associate underlines with "inserted text" - to me it just means "highlight"/"emphasize".
A short dive into Wikipedia about underscore:
In proofreading, underscoring is a convention that says "set this text in italic type"
https://en.wikipedia.org/wiki/Underscore