What problem does this address?

The editor toolbar offers a lot of formatting options for content within a Richtext field:

However, sometimes we want to be able to apply inline formatting for which there is no option in the Toolbar. For instance let's say we want a single h1 tag that looks like this:

In the editor, I can make the text fit on two lines by using shift-enter and can make the top line bold via the Toolbar, but there isn't a way to provide a different font size inline:

In general, this is fine. I don't expect the Toolbar to offer every possible formatting option just like I don't expect block controls to offer every possible option either. However, on the block level, we can add custom classes so that we can write our own custom CSS at-will:

There is currently no way to do that via the editor interface for a section of inline text.

What is your proposed solution?

In a perfect world, I would love an additional option in the Toolbar that would provide an input field (similar to the ability to add a link to an arbitrary selection of text). When highlighting text and providing this input field with a class name, it would wrap that text in a span tag with that class applied.

This is, of course, currently possible by simply swapping to the Code editor view. However, offering a Toolbar button will definitely reduce friction, just like having the ADDITIONAL CSS CLASS(ES) input field reduces friction for adding custom classes to full blocks.